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(CNN) -- Major questions hang over the investigation into a deadly accident involving an Amtrak passenger train and a tractor-trailer truck, and it could take up to a year for investigators to come up with answers, a federal official said Sunday.

At least 20 people listed on the manifest for the California Zephyr have not been accounted for by authorities, National Transportation Safety Board member Earl Weener told reporters at a news conference.

Officials have been trying to resolve the discrepancy since Friday, when a tractor-trailer truck slammed into the Chicago-to-California Amtrak train at a railroad crossing east of Reno, Nevada, killing six people.

Weener told reporters at an earlier news conference that investigators believed the discrepancy in passenger numbers arose because some people had either already gotten off the train or bought a ticket but did not take the train.

There are also troubling discoveries about the trucking company involved in the crash, John Davies Trucking of Battle Mountain, Nevada.

Records indicate that the company had 19 previous safety violations, Weener said. But he warned that further inquiry was in order, since officials didn't have the "context" to put the violations in perspective.

The NTSB is planning to visit the company this week. Investigators will review records for the company and the driver, Weener said. They will examine the driver's health and medical history as well as his training and experience, he said.

The investigation could take up to a year to complete, Weener said, adding that this is the "fact-finding" portion of the inquiry.

The trucking company, which advertises itself as a family-owned business that hauls concrete, did not immediately respond to a CNN request for comment.

"An investigation of this sort has many different facets," Weener said. "As we've gotten into the investigation we've realized there are other skills and specialties that we need, such as fire inspectors ... there are just a lot of loose ends that you have to tie up as fast as you can."

Weener said portions of the truck are still embedded in a rail car, making it difficult for investigators to do an assessment.

Authorities do not believe the accident was caused by a problem with the safety measures at the intersection where the accident occurred. It has cantilevers, lights, crossing gates, signs and crossbar markings on the pavement, Weener said. Additionally, the light that warns of an approaching train is set to flash for a period that's longer than the federal standard.

The 10-car train was carrying 204 passengers and 14 crew members when the accident happened, according to an Amtrak statement released Friday.

Skid marks show the driver of the truck involved in the crash slammed on his brakes and slid more than 300 feet before hitting the train, Weener said at an earlier news conference. The slide sparked a fire that burned the truck and two train cars, he said.

CNN's Kim Hutcherson contributed to this report