A Nevada trucking company under scrutiny for a fiery crash with an Amtrak train that left at least six dead has been cited repeatedly by state authorities for crashes, unsafe driving and operating a truck with tires so bald the vehicle had to be taken off the road.

A driver working for John Davis Trucking Co. of Battle Mountain smashed through crossing gates and into two double-decker cars of an Amtrak train Friday, killing the driver and five people on the train.

Federal records reviews by The Associated Press on Sunday show the Nevada Department of Public Safety cited the company for two crashes in the last two years, including one in February 2010 that injured a person in Washoe County.

In a January inspection, authorities found tires on one company tractor-trailer so bald they deemed the rig an imminent hazard to public safety.

Meanwhile, Nevada Highway Patrol Trooper Chuck Allen said Sunday that officials for the Churchill County Coroner Office continued to sift through the rubble of two rail cars gutted by the fire that burned extremely hot. He said they have asked at least one forensic anthropologist to assist them late Sunday or Monday.

"I think it was so hot that they want to make sure they are not missing anything," Allen told AP. "They want to figure out if there are any more bodies and if so, how many. They want to rule out the possibility that yes there are more or that no, there are not."

Investigators also continued on Sunday 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.

"Just from handling or being on the scene of so many accidents, there are so many alternatives to consider," Allen said.

"Not necessary just drugs or alcohol, but fatigue, driver inattention. Did he have a CB (citizen band radio)? Was he talking to his buddies behind him? If so, was he looking in the side view mirror and not looking at the road ahead?" he said. "I don't think we'll ever know for sure."

National Transportation Safety Board member Earl Weener said Sunday that the flashing lights at the crossing guard, which are set to blink for 25 seconds before a train approaches, would have been visible from a half-mile away if a motorist was driving at the highway's 70 mph speed limit.

The investigation has not yet revealed how fast the driver, who was approaching the crossing from the south, might have been going, Weener said.

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 the train before noon on Friday.

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.

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 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."

Weener said Saturday that 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."

Weener said investigators looking into the driver's performance would meet with John Davis Trucking Co. on Tuesday and review the driver's medical history, training and experience. He also has said the driver's professional commercial driving record "is an area we will be taking a very close look at."

The driver's name has not been released.

John Davis has 67 trucks and 130 drivers, who drove nearly 4 million miles within Nevada state lines in 2008, the most recent data available from a database of inspections and crashes involving large commercial vehicles kept by the Federal Motor Carrier Safety Administration.

John Davis' website describes it as a family-owned trucking company that owns more than 100 vehicles and a concrete plant, and specializes in hauling sand and gravel as well as ore to and from the local area mines.

The company did not immediately return a call or email seeking comment on Sunday.

On Jan. 19, during a roadside inspection Nevada authorities found tires on one of the company's tractor-trailers were so bald the threads were exposed, and ordered it to immediately be taken off the road because it was an imminent hazard to public safety, records show.

In the last two years, Nevada authorities also cited the company for 16 other vehicle maintenance violations, including oil leaks and inoperative lamps, but no others were deemed sufficiently serious to order the vehicle off the road.

Weener said the company had received seven violations since 2010. None of those violations pertained to the truck involved in the Amtrak wreck about 70 miles east of Reno.

On Feb. 3, 2010, the Nevada Highway Patrol reported a company tractor-trailer driver got in a crash in Washoe County that left one person injured, and either one or both of the vehicles was towed.

On Oct. 29, 2009, crash reports show another company driver got in a wreck in Humboldt County, in which one or both of the vehicles had to be towed away.

Overall, large-truck crashes in Nevada caused 199 injuries last year, but federal records don't detail who was at fault.

At a roadside inspection on Feb. 22, inspectors cited a John Davis driver for failing to use a seat belt and for an equipment defect on the rig's hydraulic brake system. In another on April 5, 2010, Nevada authorities cited another driver for lane restriction violations.

The company also was cited for cargo violations that included prohibited hazardous material markings on packages one driver was carrying and labeling problems, the records show.

Allen said it is not unusual for state public safety officials doing spot roadside inspections to take trucks out of service for unsafe driving practices of discrepancies in travel logs. He said he wasn't familiar with the trucking company's record but said that "having just a couple of tickets, I don't think is an alarming issue."

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Burke reported from San Francisco