SAN FRANCISCO — Frank Flores grew up in rough East Los Angeles and became a police officer who now serves as a gang expert, helping to put away the kind of thugs that wreaked havoc on his childhood neighborhood.

He’s the type of witness a jury surely would like. And he’s the type prosecutors desperately want to help prove the rackeetering charges at the heart of the giant federal gang case that, so far, hasn’t yielded much for the government.