CHICAGO (AP) — Federal officials are investigating what caused a Chicago commuter train to jump its track and crash up an escalator at one of the world's largest airports.

National Transportation Safety Board officials will be back Tuesday at O'Hare International Airport, where a Chicago Transit Authority train crashed early Monday morning at the airport stop.

More than 30 people suffered non-life-threatening injuries.

Transportation expert Joseph Schwieterman says it's "a miracle that nobody died."

He says the accident likely would have been far worse had it happened during the day, when the trains are often full and the escalator packed with people heading into O'Hare.

The union representing the operator says she may have dozed off. The NTSB hasn't drawn conclusions, saying it will scrutinize the train's breaks, signals and other potential factors.