Whose gun was used in the Steinle shooting? Was it from law enforcement?

Yes. The gun belonged to the Bureau of Land Management and was stolen from a vehicle parked in the city, IIRC a couple days before. IIRC, the prosecution did not believe that Zarate, the man who they charged with Steinles murder, had stolen the gun. Zarate claimed that he had found the gun discarded on the pier where Steinle was killed and that when he picked it up to check it out it accidentally discharged.

A stolen FBI gun.