curl -sS 'https://matrix.example.com/_synapse/admin/v1/rooms/!abc123:example.com/context/$143273582443PhrSn:example.org' -H "Authorization: Bearer $ACCESS_TOKEN"

Returns the target event (identified by its $ event ID) plus events before and after it, giving context for abuse investigations. Also includes relevant state like the room name and join rules at that point. Pair with the event_reports endpoints to review reported messages in situ.