Two truck drivers and a train engineer watched helplessly as a semitrailer skidded 100 yards (meters) before it smashed through crossing gates and into two double-decker cars of an Amtrak train at a Nevada highway crossing.

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. 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 late Friday morning. Authorities earlier said the truck's driver was among the dead, and a transportation union confirmed that number included one of its members, the train's conductor.

Weener said 28 people were unaccounted for, but that the figure was "spongy" because some passengers may have left 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 on its website that 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 that the crossing gates and warning lights were working.

The train's engineer saw the truck approaching the crossing about 70 miles (110 kilometers) 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 (125 kph) in an 80-mph (130-kph) zone, traveled another half mile (kilometer) 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 on Sunday, partly to search for additional possible victims, as well as to try to rectify discrepancies in the passenger manifest.

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AP writers Martin Griffith in Reno and Amanda Kwan in Phoenix contributed to this report.