OKLAHOMA CITY — The remains of three Union Pacific crew members killed when two trains crashed in a fiery head-on collision in the Oklahoma Panhandle have been recovered, the state’s medical examiner’s office said Tuesday.

The “very badly burned” remains have been sent to the medical examiner’s office in Oklahoma City, spokeswoman Amy Elliott said.

The Union Pacific trains slammed into each other just east of Goodwell on Sunday morning, triggering a diesel-fueled fireball that appeared to weld the locomotives together. Of the four rail workers on the trains at that time, one conductor managed to jump free before the crash. He suffered only cuts and bruises.

The National Transportation Safety Board has said it appears signals were working properly at the time of the wreck, and that one of two trains passing through the flat landscape should have pulled onto a side track. The NTSB said there was “no survivable space” in the locomotives’ cabins following the collision.

The United Transportation Union identified those aboard the trains as conductor Brian L. Stone, 50, of Dalhart, Texas; and engineers Dan Hall and John Hall. The conductor who escaped virtually unharmed is Juan Zurita. The Halls were not related.

Federal investigators said they want to know why one of the trains failed to pull into a side track as the other train approached on the main line.

“One train had the right of way,” NTSB member Mark Rosekind said Monday night. “We’re still getting the data to figure out what was scheduled to happen. ... who was supposed to be where and when.”

The NTSB planned to interview Zurita as part of its investigation.

On Tuesday, former Federal Railroad Administration official Gil Carmichael said the likely cause of the crash was human error. He said that if the signals were working properly, crew members were likely at fault.

A witness to the accident, truck driver Gary Mathews, of Independence, Mo., said neither train blew a horn or signaled with lights as they barreled toward each other over the stark Panhandle landscape.

“I was thinking, ‘I’m going to see a train crash unless somebody does something,”’ Mathews told The Associated Press.

An early review found no problems with the signal system along the tracks near Goodwell, 300 miles northwest of Oklahoma City, Rosekind said. The track, too, appeared normal. The NTSB will check phone records to ensure that workers were not distracted from their duties by cellphones, and the agency also hoped to analyze data recorders similar to those found aboard airplanes, he said.

The eastbound train, hauling mixed goods from Los Angeles to Chicago, had three lead locomotives and one following. The westbound, taking cars and trucks from Kansas City to Los Angeles, was pulled by two locomotives and pushed by one.

Video was recovered from the rear locomotives, and the remnants of what is believed to be one of the so-called black box data recorders has been pulled from one train.

“Those are critical to our investigations. We can ... virtually see what happened,” Rosekind said in a telephone interview from Guymon.

Rosekind said the trains’ brakes appeared normal and no cellphones were found in the wreckage. The NTSB was checking the crew members’ recent work schedules and rest periods, and also their evaluations, he said. It was also looking into the track’s speed rating after a cross-country truck driver said he was “pacing” the train at 68 mph shortly before the crash. Freight can travel at speeds of up to 80 mph, but only on tracks with the highest ratings for cargo. Passenger trains can travel faster on higher quality rails.

The board could release a preliminary report within two weeks, though it could be a year before a final report is available, Rosekind said.

Mathews said he was traveling from Phoenix to Missouri along U.S. 54 beside the ribbon of rail. He said the westbound train seemed to slow considerably before the crash, but that the eastbound was still traveling at “65 or better” when the trains hit about 50 yards from him, he said.

“A blast of hot air came through the side glass, and it put a burn on you like you step out of an air-conditioned bar into 110 degrees, through the glass,” he said. “There was a thud and it was over. Smoke was rolling. Smoke went up so high it was like a foundry on fire, and it was barreling straight up,” he recalled.

“After I seen it, the feeling went through me, it scared the tar out of me, and I didn’t stop until I reached Emporia” in Kansas, about 350 miles away. He was interviewed Sunday by the Guymon Daily Herald newspaper and reached Monday by the AP.

“I didn’t even stop,” Mathews said, as he recalled watching the scene unfold in his rearview mirror.

Oklahoma Highway Patrol Trooper Betsy Randolph said Tuesday that she was surprised that any remains were found because of the intensity of the fire.

“I’ve never seen anything like it,” said Randolph, a 17-year veteran of the OHP. “I would liken it to some of those way overdone Hollywood stunt scenes.”

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Associated Press writer Justin Juozapavicius contributed to this report from Tulsa, Okla.