BOSTON — In 1990, Richard Abath was a 23-year-old self-described hippie and guitarist for a rock band named Ukiah who, while moonlighting as a security guard, allowed two thieves posing as police officers to enter the Isabella Stewart Gardner Museum here. For 25 years, his decision to buzz in the men who carted away $500 million worth of art has been largely treated as the hapless act of a rookie watchman who fell for a bluff. But on Thursday, federal officials still investigating the art heist — the largest in American history — drew fresh attention to Mr. Abath’s actions...