LOS ANGELES (AP) -- Federal officials investigating a commuter rail collision that killed 25 people said they want to review cell phone records to determine if an engineer blamed for running a stop signal before the crash may have been text messaging at the time.
With the probe into Friday's crash continuing, a smaller number of commuters than normal returned to the rails Monday morning.
Mayor Antonio Villaraigosa boarded one of the morning's earliest trains.
"I want to dispel any fears about taking the train," the mayor said. "Safety has to be our number one concern, and while accidents can and do happen, taking the train is still one of the safest and fastest options for commuters."
The National Transportation Safety Board confirmed on Sunday that the engineer, who was killed in the crash, had failed to stop at the final red signal.
NTSB experts are planning to review the cell phone records of two 14-year-old boys and the engineer after the teens told CBS2-TV that they received a text message from the engineer shortly before the crash.
The Los Angeles station said the teen was among a group of youths who befriended the engineer and asked him questions about his work.
NTSB board member Kitty Higgins said investigators did not find a cell phone belonging to the engineer in the wreckage but would request his cell phone records, as well as those of the boys.
"We are going to be obtaining records from their cell phones and from the cell phones of the deceased engineer and will use our subpoena authority or whatever other legal authority we need and to begin to determine exactly what happened and what if any role that might have played in this accident," she said Sunday.
The commuter train carrying 220 people rolled past stop signals Friday and barreled head-on into a Union Pacific train in Chatsworth. The accident, the nation's deadliest rail disaster in 15 years, left train cars so mangled that some bodies had to be removed in pieces. The crash injured 138 people.
Regular riders said the number of commuters Monday was far short of a normal day.
Genise Silvers said she was on the train that crashed Friday but got off at the stop before the deadly crash. She said her daughter didn't want her to take the train to work Monday, but she felt she had to.
"I think I'm still a little in shock. I think it'll be OK. I kind of wanted to come today to be with my friends. When you've been through such a thing as this you want to commune with your friends," Silvers said at the Chatsworth Metrolink station.
Also Monday, the Metrolink spokeswoman who announced Saturday that the engineer's mistake caused the crash resigned. She said the railroad's board called her announcement "premature," even though NTSB officials later backed it up.
Metrolink did not return phone messages on the resignation.
NTSB investigators said Sunday that the train failed to stop at the final red signal, which forced the train onto a track at 42 mph where the Union Pacific freight was traveling in the opposite direction, Higgins said at a news conference. But the agency continued to investigate why that happened.
Higgins said she believed the crash could have been prevented with technology that stops a train on the track when a signal is disobeyed. The technology was not in place where the collision occurred.
"I believe this technology could have prevented the accident. If he ran the signal the train would have been stopped. I've seen it tested. It makes a difference," she said.
"What's it going to take? How many more accidents are we going to have to see like this that could have been prevented if this technology were in place?" Higgins said.
Higgins said audio recordings from the commuter train indicate a period of silence as it passed the last two signals before the fiery wreck, a time when the engineer and the conductor should have been performing verbal safety checks.
She cautioned, however, that the train may have entered a dead zone where the recording was interrupted.
Higgins said the NTSB would measure the distance between the signals along the track on Monday. Investigators also want to interview the conductor, who was injured, about the recording, she said.
"He'll be able to tell us whether he recalls the engineer calling out and him confirming those signals," Higgins said.
Data show that the Metrolink train ran the red light signal with devastating consequences.
"The Metrolink train went through the signal, did not observe the red signal and essentially forced open this section of the switch," Higgins said. "The switch bars were bent like a banana. It should be perfectly straight."
Higgins said experts still must examine whether the signal was working properly and were in the Metrolink engineer's line of sight.
However, she stressed that obeying signals on the track was an engineer's responsibility at the helm of a train.
"My understanding is it is very unusual for an experienced engineer to run a red light," she said.
Metrolink said earlier Sunday that a dispatcher tried to warn the engineer of the commuter train that he was about to collide with a freight train but the call came too late. The dispatcher reached the conductor in the rear of the train, but by then it had already crashed into the oncoming Union Pacific train, Metrolink officials said.
However, the NTSB contradicted Metrolink's report. Higgins said that the dispatcher noticed something was wrong, but before he could contact the train, the conductor - who survived - called in to report the wreck.
The collision occurred on a horseshoe-shaped section of track in Chatsworth at the west end of the San Fernando Valley, near a 500-foot-long tunnel underneath Stoney Point Park.
The commuter train was heading from Union Station in downtown Los Angeles to Ventura County. The impact rammed the Metrolink engine backward, jamming it deep into the first passenger car.
It was the deadliest passenger train crash since Sept. 22, 1993, when Amtrak's Sunset Limited plunged off a trestle into a bayou near Mobile, Ala., moments after the trestle was damaged by a towboat; 47 people were killed.
Associated Press writers Thomas Watkins, Michael Blood, Daisy Nguyen, Christina Hoag, Greg Risling, Justin Pritchard, James Beltran, John Rogers, Shaya Tayefe Mohajer and Gillian Flaccus contributed to this report.