How to find logs from system crashes?

[jds]

My OPNsense box has started crashing (seemingly) randomly every couple of days. It is difficult to debug without logs from the time of the crash.  When it crashes there is no responsiveness from pings or ssh or webUI, so I have to reboot.  However, after I reboot, all the logs I can find (e.g., /var/log/system) are from times after the reboot.

How can I get info from the crash to debug?  Any help appreciated.