How can I investigate the root cause of a fatal error on a Linux system potentially compromised by a hacker?

Investigate using these steps: 1) Examine system logs (syslog, auth.log, kernel.log) for suspicious activity leading up to the error. 2) Check for unauthorized file modifications using file integrity monitoring tools (AIDE, Tripwire). 3) Analyze process listings for unusual or unknown processes consuming excessive resources. 4) Inspect network connections for rogue connections to external IPs using tools like `netstat` or `ss`. 5) Consider using memory forensics if you suspect rootkit activity. Take a memory dump before rebooting.