Officials have raised the death toll to six in a fiery truck-Amtrak train crash east of the Reno, Nev.
The truck plowed into the side of train Friday afternoon in a moment of sudden impact that set off a huge fireball.
In the last several days, investigators combed through the charred and gutted wreckage looking for clues.
Today, two forensic experts - forensic anthropologists- will work with the medical examiner to find out if there are more victims.
Investigators also plan to review the background of the tractor trailer driver.
"Investigators will look at the truck driver's driving and medical records, as well as autopsy results to determine whether any drugs were involved," National Transportation Safety Board spokesman Peter Knudson told ABC affiliate KOLO-TV in Reno.
"They'll also check to make sure railroad lights and crossing gates were working," he added. According to the Nevada State Police that the driver tried to slow down at the last minute but was unable to stop.
Officials said they found extensive skidmarks from the truck, indicating the driver had slammed on the breaks.
The preliminary investigation and witness statements indicate the truck driver was going northbound on U.S. Interstate 95 and the gates were operational and flashing before the collision. Witnesses said it appeared that the driver tried to slow down at the last minute but couldn't.
What remains confusing is that witnesses indicate that at least the signal gates there were working, although it's unclear whether the signal lights were working as well, investigators said.
While the cause of the crash is still being investigated, Trooper Chuck Allen said visibility wasn't an issue though it's unclear if the truck driving above the 70 miles per hour speed limit for that stretch of Interstate 95.
The NTSB dispatched a "go-team" to investigate Friday's accident.
Searching for Clues
On Friday, investigators had trouble getting access to the smoldering rail cars to discover if they could locate additional survivors or victims.
Among the dead was 68-year-old conductor Lorette Lee, who came from a long line of railway workers.
Nevada Highway Patrol Trooper Dan Lopez told ABC News on Friday that 18 to 20 people were transported to hospitals in buses for treatment.
Local media reported "dozens" of people were hurt, including some carried in helicopters to trauma centers.
Amtrak spokesman Marc Magliari in Oakland, Calif., told The Associated Press that 204 passengers and 14 crew members were aboard the California Zephyr en route from Chicago to Emeryville, Calif., which is about 300 miles west of the crash site.
Passenger Celia Levaca was with her young son in the train car that the truck hit.
"My son's the only thing I grabbed when everything happened," Levaca said. "Everything happened so fast."
Two of the rail cars caught fire as passersby on the highway stopped to help. Local and state police and rescue squads rushed to the scene. A nearby Naval air station also sent helicopters to help evacuate the injured.
The accident at 11:20 a.m. PT Friday shut down a section of U.S. 95 between Interstate 80 and Fallon, Nev.
Amtrak set up a phone number where friends and family of people aboard the train could seek information -- 800-523-9101.
Collisions at rail crossings are not uncommon. Safety groups estimate that a train in the United States collides with a person or vehicle nearly every three hours.
In 2010 there were more than 2,000 collisions between vehicles and trains at railroad crossings, according to Operation LifeSafer.
ABC News Radio and The Associated Press contributed to this report.