It stretches credibility but here are some ways he could have not been directly involved:
- he lends the gun to someone, they shot the couple, that person files off the serial number and returns the gun to LH. Filing off a serial number should have been plainly visible so LH should have said "why did you damage my gun" (just like you'd ask what happened if you lent someone a drill or tool and it came back damaged.)
- he lends the gun to someone, they shot the couple, the gun is returned but LH figures out what happened and files off the serial #.
- gun was stolen, and somehow gets back into his possession even though the serial # is how guns are legally traced and returned to burglary victims.
Did the police ever say if LH said "this person had the gun, wasn't me" but they couldn't make a case? It certainly sounds like they couldn't make a case against LH even though he seems like an obvious suspect.