Federal agents are reportedly opening a criminal investigation into the boat fire that killed 34 people off the California coast.

The LA Times says the Coast Guard and the US Attorney’s office in Los Angeles are heading up the probe. The cause of the Labor Day fire remains unknown, but the feds are apparently looking into possible safety violations. Officials have previously said the diving boat lacked a required night watchman when the fire broke out before dawn. Federal agents spent the weekend searching the office of the company that owned the boat.