diff --git a/app/(protected)/hse/incidents/[id]/investigation/page.tsx b/app/(protected)/hse/incidents/[id]/investigation/page.tsx
new file mode 100644
index 0000000..0994108
--- /dev/null
+++ b/app/(protected)/hse/incidents/[id]/investigation/page.tsx
@@ -0,0 +1,54 @@
+export const dynamic = 'force-dynamic'
+
+import { notFound, redirect } from 'next/navigation'
+import Link from 'next/link'
+import { createClient } from '@/lib/supabase/server'
+import { InvestigationForm } from '@/components/incidents/investigation-form'
+
+interface Props {
+  params: Promise<{ id: string }>
+}
+
+export default async function InvestigationPage({ params }: Props) {
+  const { id } = await params
+  const supabase = await createClient()
+
+  const { data, error: authError } = await supabase.auth.getUser()
+  if (authError || !data?.user) redirect(`/login?redirect=/hse/incidents/${id}/investigation`)
+
+  const { data: profile } = await supabase
+    .from('users').select('role').eq('id', data.user.id).single()
+  if (!profile || profile.role !== 'hse') redirect('/hse/incidents')
+
+  const { data: incident } = await supabase
+    .from('incidents')
+    .select('id, reference_no, status')
+    .eq('id', id)
+    .single()
+
+  if (!incident) notFound()
+  if (!['triaged', 'investigating'].includes((incident as { status: string }).status))
+    redirect(`/hse/incidents/${id}`)
+
+  const { data: existing } = await supabase
+    .from('investigations')
+    .select('id')
+    .eq('incident_id', id)
+    .order('created_at', { ascending: false })
+    .limit(1)
+    .maybeSingle()
+
+  return (
+    <main className="max-w-2xl mx-auto px-4 py-6">
+      <Link href={`/hse/incidents/${id}`} className="text-sm text-blue-600 hover:underline mb-4 inline-block">
+        ← Back to incident
+      </Link>
+      <h1 className="text-xl font-bold text-gray-900 mb-1">Investigation Workspace</h1>
+      <p className="text-sm text-gray-500 mb-6">{(incident as { reference_no: string | null }).reference_no ?? id}</p>
+      <InvestigationForm
+        incidentId={id}
+        existingInvestigationId={(existing as { id: string } | null)?.id ?? null}
+      />
+    </main>
+  )
+}
diff --git a/app/api/incidents/[id]/investigation/route.ts b/app/api/incidents/[id]/investigation/route.ts
new file mode 100644
index 0000000..363ff75
--- /dev/null
+++ b/app/api/incidents/[id]/investigation/route.ts
@@ -0,0 +1,112 @@
+export const dynamic = 'force-dynamic'
+
+import { NextRequest, NextResponse } from 'next/server'
+import { createClient } from '@/lib/supabase/server'
+
+export async function POST(
+  request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const { id } = await params
+  const supabase = await createClient()
+
+  const { data: { user }, error: authError } = await supabase.auth.getUser()
+  if (authError || !user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+
+  const { data: profile } = await supabase
+    .from('users').select('role').eq('id', user.id).single()
+  if (!profile || profile.role !== 'hse')
+    return NextResponse.json({ error: 'Forbidden' }, { status: 403 })
+
+  const { data: incident } = await supabase
+    .from('incidents').select('status').eq('id', id).single()
+  if (!incident) return NextResponse.json({ error: 'Not found' }, { status: 404 })
+  if (incident.status !== 'triaged')
+    return NextResponse.json({ error: 'Incident must be triaged first' }, { status: 409 })
+
+  const body = await request.json()
+  const method: 'five_why' | 'fishbone' | 'other' = body.method ?? 'five_why'
+
+  const { data: inv, error } = await supabase
+    .from('investigations')
+    .insert({
+      incident_id: id,
+      investigator_id: user.id,
+      method,
+      findings_text: body.findings_text ?? null,
+      root_cause_summary: body.root_cause_summary ?? null,
+      five_why_steps: method === 'five_why' ? (body.five_why_steps ?? []) : null,
+      fishbone_categories: method === 'fishbone' ? (body.fishbone_categories ?? []) : null,
+    })
+    .select('id')
+    .single()
+
+  if (error || !inv) return NextResponse.json({ error: 'Insert failed' }, { status: 500 })
+
+  await supabase
+    .from('incidents')
+    .update({ status: 'investigating' })
+    .eq('id', id)
+
+  await supabase.rpc('write_audit_log', {
+    p_table_name: 'incidents',
+    p_record_id: id,
+    p_action: 'investigation_started',
+    p_new_value: { status: 'investigating', investigation_id: inv.id },
+  })
+
+  return NextResponse.json({ id: inv.id })
+}
+
+export async function PATCH(
+  request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const { id } = await params
+  const supabase = await createClient()
+
+  const { data: { user }, error: authError } = await supabase.auth.getUser()
+  if (authError || !user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+
+  const { data: profile } = await supabase
+    .from('users').select('role').eq('id', user.id).single()
+  if (!profile || profile.role !== 'hse')
+    return NextResponse.json({ error: 'Forbidden' }, { status: 403 })
+
+  const body = await request.json()
+  const { investigation_id, complete, ...fields } = body
+
+  if (!investigation_id) return NextResponse.json({ error: 'investigation_id required' }, { status: 422 })
+
+  const updateData: Record<string, unknown> = {
+    findings_text: fields.findings_text ?? null,
+    root_cause_summary: fields.root_cause_summary ?? null,
+    five_why_steps: fields.five_why_steps ?? null,
+    fishbone_categories: fields.fishbone_categories ?? null,
+  }
+  if (complete) updateData.completed_at = new Date().toISOString()
+
+  const { error } = await supabase
+    .from('investigations')
+    .update(updateData)
+    .eq('id', investigation_id)
+    .eq('incident_id', id)
+
+  if (error) return NextResponse.json({ error: 'Update failed' }, { status: 500 })
+
+  if (complete) {
+    await supabase
+      .from('incidents')
+      .update({ status: 'capa_pending' })
+      .eq('id', id)
+
+    await supabase.rpc('write_audit_log', {
+      p_table_name: 'incidents',
+      p_record_id: id,
+      p_action: 'investigation_completed',
+      p_new_value: { status: 'capa_pending' },
+    })
+  }
+
+  return NextResponse.json({ ok: true })
+}
diff --git a/components/incidents/investigation-form.tsx b/components/incidents/investigation-form.tsx
new file mode 100644
index 0000000..5fda866
--- /dev/null
+++ b/components/incidents/investigation-form.tsx
@@ -0,0 +1,212 @@
+'use client'
+
+import { useState } from 'react'
+import { useRouter } from 'next/navigation'
+
+type FiveWhyStep = { why: string; answer: string }
+type FishboneCategory = {
+  category: 'man' | 'machine' | 'method' | 'material' | 'environment' | 'measurement'
+  causes: string[]
+}
+
+const FISHBONE_CATEGORIES: FishboneCategory['category'][] = [
+  'man', 'machine', 'method', 'material', 'environment', 'measurement',
+]
+const CATEGORY_LABELS: Record<FishboneCategory['category'], string> = {
+  man: 'Man (People)',
+  machine: 'Machine',
+  method: 'Method',
+  material: 'Material',
+  environment: 'Environment',
+  measurement: 'Measurement',
+}
+
+interface Props {
+  incidentId: string
+  existingInvestigationId: string | null
+}
+
+export function InvestigationForm({ incidentId, existingInvestigationId }: Props) {
+  const router = useRouter()
+  const [method, setMethod] = useState<'five_why' | 'fishbone' | 'other'>('five_why')
+  const [fiveWhy, setFiveWhy] = useState<FiveWhyStep[]>([
+    { why: 'Why did the incident happen?', answer: '' },
+  ])
+  const [fishbone, setFishbone] = useState<FishboneCategory[]>(
+    FISHBONE_CATEGORIES.map(c => ({ category: c, causes: [''] }))
+  )
+  const [findingsText, setFindingsText] = useState('')
+  const [rootCause, setRootCause] = useState('')
+  const [complete, setComplete] = useState(false)
+  const [saving, setSaving] = useState(false)
+  const [error, setError] = useState<string | null>(null)
+
+  function addWhyStep() {
+    if (fiveWhy.length >= 5) return
+    setFiveWhy([...fiveWhy, { why: '', answer: '' }])
+  }
+  function updateWhyStep(i: number, field: keyof FiveWhyStep, value: string) {
+    setFiveWhy(fiveWhy.map((s, idx) => idx === i ? { ...s, [field]: value } : s))
+  }
+  function updateFishboneCause(catIdx: number, causeIdx: number, value: string) {
+    setFishbone(fishbone.map((c, i) =>
+      i === catIdx ? { ...c, causes: c.causes.map((cause, j) => j === causeIdx ? value : cause) } : c
+    ))
+  }
+  function addFishboneCause(catIdx: number) {
+    setFishbone(fishbone.map((c, i) => i === catIdx ? { ...c, causes: [...c.causes, ''] } : c))
+  }
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault()
+    setSaving(true)
+    setError(null)
+
+    const payload = {
+      method,
+      findings_text: findingsText || null,
+      root_cause_summary: rootCause || null,
+      five_why_steps: method === 'five_why' ? fiveWhy.filter(s => s.answer) : null,
+      fishbone_categories: method === 'fishbone'
+        ? fishbone.map(c => ({ ...c, causes: c.causes.filter(Boolean) })).filter(c => c.causes.length > 0)
+        : null,
+    }
+
+    let res: Response
+    if (existingInvestigationId) {
+      res = await fetch(`/api/incidents/${incidentId}/investigation`, {
+        method: 'PATCH',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ ...payload, investigation_id: existingInvestigationId, complete }),
+      })
+    } else {
+      res = await fetch(`/api/incidents/${incidentId}/investigation`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(payload),
+      })
+    }
+
+    if (!res.ok) {
+      const data = await res.json()
+      setError(data.error ?? 'Save failed')
+      setSaving(false)
+      return
+    }
+    router.push(`/hse/incidents/${incidentId}`)
+    router.refresh()
+  }
+
+  return (
+    <form onSubmit={handleSubmit} className="space-y-6">
+      <div>
+        <label className="block text-sm font-medium text-gray-700 mb-1">RCA Method</label>
+        <div className="flex gap-3">
+          {(['five_why', 'fishbone', 'other'] as const).map(m => (
+            <button
+              key={m} type="button"
+              onClick={() => setMethod(m)}
+              className={`px-3 py-1.5 rounded-lg text-sm font-medium border ${
+                method === m
+                  ? 'bg-blue-600 text-white border-blue-600'
+                  : 'bg-white text-gray-700 border-gray-300 hover:border-blue-400'
+              }`}
+            >
+              {m === 'five_why' ? '5-Why' : m === 'fishbone' ? 'Fishbone' : 'Other'}
+            </button>
+          ))}
+        </div>
+      </div>
+
+      {method === 'five_why' && (
+        <div className="space-y-3">
+          <p className="text-sm font-medium text-gray-700">5-Why Analysis</p>
+          {fiveWhy.map((step, i) => (
+            <div key={i} className="border border-gray-200 rounded-lg p-3 space-y-2">
+              <label className="block text-xs text-gray-500">Why #{i + 1}</label>
+              <input
+                type="text" value={step.why}
+                onChange={e => updateWhyStep(i, 'why', e.target.value)}
+                placeholder="Why did this happen?"
+                className="w-full rounded border border-gray-300 px-2 py-1.5 text-sm"
+              />
+              <textarea
+                value={step.answer}
+                onChange={e => updateWhyStep(i, 'answer', e.target.value)}
+                rows={2} placeholder="Answer / finding…"
+                className="w-full rounded border border-gray-300 px-2 py-1.5 text-sm"
+              />
+            </div>
+          ))}
+          {fiveWhy.length < 5 && (
+            <button type="button" onClick={addWhyStep}
+              className="text-sm text-blue-600 hover:underline">
+              + Add another Why
+            </button>
+          )}
+        </div>
+      )}
+
+      {method === 'fishbone' && (
+        <div className="space-y-3">
+          <p className="text-sm font-medium text-gray-700">Fishbone (Ishikawa) Analysis</p>
+          {fishbone.map((cat, catIdx) => (
+            <div key={cat.category} className="border border-gray-200 rounded-lg p-3">
+              <p className="text-xs font-semibold text-gray-600 mb-2">{CATEGORY_LABELS[cat.category]}</p>
+              {cat.causes.map((cause, causeIdx) => (
+                <input
+                  key={causeIdx}
+                  type="text" value={cause}
+                  onChange={e => updateFishboneCause(catIdx, causeIdx, e.target.value)}
+                  placeholder="Contributing cause…"
+                  className="w-full rounded border border-gray-300 px-2 py-1.5 text-sm mb-1.5"
+                />
+              ))}
+              <button type="button" onClick={() => addFishboneCause(catIdx)}
+                className="text-xs text-blue-600 hover:underline">
+                + Add cause
+              </button>
+            </div>
+          ))}
+        </div>
+      )}
+
+      <div>
+        <label className="block text-sm font-medium text-gray-700 mb-1">Key Findings</label>
+        <textarea
+          value={findingsText}
+          onChange={e => setFindingsText(e.target.value)}
+          rows={3}
+          className="w-full rounded-lg border border-gray-300 px-3 py-2 text-sm"
+          placeholder="Describe the sequence of events and contributing factors…"
+        />
+      </div>
+
+      <div>
+        <label className="block text-sm font-medium text-gray-700 mb-1">Root Cause Summary</label>
+        <textarea
+          value={rootCause}
+          onChange={e => setRootCause(e.target.value)}
+          rows={2}
+          className="w-full rounded-lg border border-gray-300 px-3 py-2 text-sm"
+          placeholder="One-sentence root cause statement…"
+        />
+      </div>
+
+      {!existingInvestigationId && (
+        <label className="flex items-center gap-2 text-sm text-gray-700">
+          <input type="checkbox" checked={complete} onChange={e => setComplete(e.target.checked)}
+            className="rounded border-gray-300 text-blue-600" />
+          Mark investigation complete (transitions incident to CAPA Pending)
+        </label>
+      )}
+
+      {error && <p className="text-sm text-red-600">{error}</p>}
+
+      <button type="submit" disabled={saving}
+        className="w-full bg-indigo-600 text-white rounded-lg py-2 text-sm font-semibold disabled:opacity-50">
+        {saving ? 'Saving…' : existingInvestigationId ? 'Update Investigation' : 'Start Investigation'}
+      </button>
+    </form>
+  )
+}