Bank robbers in the movies routinely wear masks to hide their identities. So maybe Justice Department lawyers should have worn Barack Obama and former Attorney General Loretta Lynch masks this month when they announced their latest bank stickup.

That would have been fitting given the 202-page lawsuit, filed in New York federal court, that accuses the Swiss bank UBS of misleading investors about the risks of mortgage-backed securities before the financial panic of 2008. The case rests on an avant-garde legal interpretation...