(CNN) -- A federal investigator said he's never seen a train climb an escalator like the one that jumped the tracks at Chicago's O'Hare International Airport early Monday.
The crash of the Chicago Transit Authority Blue Line train injured 32 people, although none seriously, according to Chicago Fire officials.
The eight-car train failed to stop at the end of the line when it arrived at the airport station at 2:52 a.m., a Chicago Fire official said. The lead car appeared to have climbed an escalator adjacent to the passenger platform.
"I've investigated many accidents and trains do different things," National Transportation Safety Board investigator Tim DePaepe told reporters. "It's all about kinetic force. I have not seen an accident like this personally."
NTSB experts will examine video from station cameras and one on the front of the train for clues to the cause of the crash, DePaepe said.
A union official told reporters Monday that the train's driver may have fallen asleep, CNN affiliate WLS reported.
"There are indications that she dozed off, yes," Amalgamated Transit Union Local 308 President Robert Kelly said, according to WLS.
Almost all of the 32 people taken to four hospitals had been released by the afternoon, he said.
Investigators are still waiting for the train operator to be released from the hospital so that they can talk to her, he said.
The wrecked train will not be moved at least until Tuesday so that investigators can examine it, DePaepe said.
Shuttle buses are carrying airport passengers to and from a nearby Blue Line station until then, a CTA official said.
CNN's Jennifer Feldman contributed to this report.