HOUSTON (AP) — Federal officials started their investigation Saturday in the collapse of one of the nation’s largest mobile cranes, which toppled at a Houston oil refinery, killing four workers and injuring seven others.

The 30-story-tall crane, capable of lifting 1 million pounds, crashed to the ground Friday at a LyondellBasell refinery in southeast Houston.

Representatives of the Occupational Safety and Health Administration began their formal accident investigation Saturday morning, said David Roznowski, a spokesman for the LyondellBasell refinery. OSHA investigators are working with the refinery, a subcontractor and the project manager.

‘‘It really is too early to say what happened,’’ Roznowski told The Associated Press on Saturday. ‘‘With the formal incident investigation, that’s where we will start to get answers, but it’s going to take time. We want to make sure no stone is left unturned and that this kind of thing doesn’t happen again.’’

The owner of the crane, Deep South Crane & Rigging of Baton Rouge, La., plans to work with the federal investigators.