SPARKS, Nev. — Two truck drivers and a train engineer watched helplessly as a semitrailer skidded the length of a football field before it smashed through crossing gates and into two double-decker cars of an Amtrak train at a highway crossing, killing at least six people.

The drivers were part of a three-truck convoy that saw the gates come down and the warning lights go off as the California Zephyr approached, National Transportation Safety Board member Earl Weener said Saturday.

They stopped, but the driver of the big rig in the lead did not, he said.

Investigators on Sunday planned to look over the scene for any clues as to why the truck driver plowed through the railroad crossing on Friday. It’s expected to take up to a year to pinpoint the cause of the crash.

The Churchill County Sheriff’s office said Saturday that six people had died in the crash. The truck driver and train conductor were among the dead.

The conductor, 68-year-old Laurette Lee of South Lake Tahoe, Calif., came from a railroad family. Her great-grandfather and grandfather worked for railroad companies, her brother is an Amtrak dispatcher and her nephew is an Amtrak conductor, Lee’s friends and family told the San Jose Mercury News.

Weener said 28 people were unaccounted for in the crash, but that the figure was “spongy” because some passengers may have gotten off the train before the crash or walked away from the scene without checking with officials.

“This is not quite like you are used to when you get on an airplane. They record exactly who gets on, and what seat they sit in,” he said. “On a train, you can get off without necessarily being tracked.”

About 20 people were injured, and the United Transportation Union said the train’s assistant conductor was among those seriously hurt. Weener said a passenger manifest counted 210 on board, but Amtrak spokeswoman Christina Leeds said 204 passengers and 14 crew members were on the train at the time.

“We are going to be working in the next several days to get more of that (unaccounted) number down the best we can,” Weener said.

At the time of the collision, Weener said visibility was excellent and the crossing gates and warning lights were working.

The train’s engineer saw the truck approaching the crossing about 70 miles east of Reno and realized the collision was inevitable, he said.

The engineer slammed on the emergency brakes, but the train, which was going about 78 mph in an 80-mph zone, traveled a half-mile more before it finally stopped, he said. The engineer watched the truck smash into two of the train’s 10 cars through the rearview mirror.

“He recalled the event clearly. He saw the truck approaching the train,” Weener said. “At some point, he knew the impact was imminent. He, in fact, watched the collision in a rearview mirror. He was hoping the train was not going to derail.”

NTSB investigators were returning to the crash site Sunday to search for more possible victims and try to rectify discrepancies in the passenger manifest.

The California Zephyr from Chicago was about 300 miles east of its destination in Emeryville, Calif., when the truck hit the two train cars, which burst into flames. Earlier witness accounts said the truck driver didn’t attempt to stop before it drove through the crossing, but Weener said the driver did try to stop because the truck skidded about 320 feet before it crashed.

The speed of the truck hasn’t been determined, but Weener said it was going “at a considerable speed” because the impact left the tractor embedded in one of the train cars.

The truck driver who died was a Nevada man in his mid-40s, Weener said. Churchill County authorities said they were working to confirm other victims’ identities and notify family members.

Weener said a team of 18 NTSB investigators were expected to remain at the scene for at least a week. NTSB spokesman Peter Knudson said they hadn’t finished combing the wreckage, and Weener said they had yet to review video data taken from the train.

“We will not be determining a probable cause of this accident while we are here,” Weener said.