It's possible, but there are a lot of "if's". You'll need to find out the details such as: disposition of the case (if any) and get all documentation.
If charges were filed, you'll need the report#, case# and court documents (if it went to court). Get all of that, a death certificate, and try to reclaim the rifle.
If he wasn't charged and there's no police report? Then you might be out of luck...think Maui PD June 2011, where a cop kept a surrendered gun.
Another thing to consider is the 20+ years...it might have been archived or disposed of by now.
It's worth a try. I'd go to the PD, ask if it's still there and what you need to do to recover it.