A fatal crash between an Amtrak train and a truck being investigated further Monday, as authorities try to account for five people still missing since Friday's fiery collision.

Six people were killed when a truck drove through closed crossing gates, ramming into the side of two train cars in the Nevada desert.

As officials continued to sift through the remains of two rail cars from the California Zephyr and the vehicle over the weekend, no more bodies have been found in the wreckage.

Nevada Highway Patrol Trooper Chuck Allen said the blaze was so hot that authorities "want to make sure they are not missing anything."  The coroner's office has asked at least one forensic anthropologist to help.

"They want to figure out if there are any more bodies and if so, how many," Allen said. "They want to rule out the possibility that, yes there are more, or that no, there are not."

The trucking company has been cited for incidents in the past, but the attention of the investigation has now been focused on the driver and the events that led up to the crash.

"Did he have a 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? I don't think we'll ever know for sure," Allen said.

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

Officials predict it will to take up to a year to pinpoint the cause of the crash.