(CNN) -- A Chicago commuter train that was parked in a service yard somehow moved onto a rail line and smashed into an oncoming train early Monday, injuring dozens of people.
How it happened is a mystery.
"I don't know the last time, if ever, that this has happened" on the Chicago Transit Authority system, spokesman Brian Steele said.
There are "more questions than answers" about the incident in Forest Park, Illinois, he said.
At least 48 people were injured; 33 of them were transported to hospitals, Forest Park Mayor Anthony Calderone told CNN affiliate WLS. They are believed to have minor injuries.
It was not immediately clear how many passengers were aboard the train.
Investigators are not characterizing the incident as a runaway train at this point, Steele said.
A central question: whether anyone had climbed on board the empty train and set it in motion.
"In order for a train to move, it has to be energized," said Ronald Ester, vice president of CTA Rail Operations.
"We call it unlocking the master controller," he said. The train would have needed to be placed in power position manually.
Authorities are looking at video feeds from the platforms and from some rail cars. They're also interviewing workers who were nearby.
It's unknown whether there were criminal activities, Steele said. Authorities did not immediately find windows broken, doors pried open or graffiti on the train.
The train that caused the accident had four cars; the one with passengers aboard had eight.
The incident took place about 8 a.m.
Train service continued but did not stop at Harlem, near the site of the crash. Shuttle buses were made available.
Steele emphasized that the CTA has "a very strong safety record."