Experts speculated Tuesday that a fiery head-on collision of two trains in the Oklahoma Panhandle was likely the result of human error, though federal investigators are still piecing together evidence and haven't determined a cause.

Three crew members were killed when the Union Pacific trains slammed into each other Sunday morning just east of Goodwell, about 300 miles northwest of Oklahoma City. The crash triggered a diesel-fueled fireball that appeared to weld the locomotives together.

The "very badly burned" remains of the victims have been sent to the medical examiner's office in Oklahoma City, agency spokeswoman Amy Elliott said Tuesday. The only other rail worker on the trains at the time of the crash managed to jump free before the collision and suffered only cuts and bruises.

The National Transportation Safety Board said it appears signals were working properly at the time of the wreck, and that one of the 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 NTSB could release a preliminary report within two weeks, though it could be a year before a final report is available, NTSB official Mark Rosekind said.

Former Federal Railroad Administration official Gil Carmichael said Tuesday that it was "very unusual" for such a collision on such flat landscape. He said it sounded like one of the crew members made an error.

Bob Jarvis, a transportation law professor at Nova Southeastern University in Fort Lauderdale, Fla., noted that human error can go beyond the train crew.

"In those kinds of accidents you really start to think about who let these two trains leave the yard. Who was monitoring? That's likely going to be human error back where the pretty lights (of a communications center) are," Jarvis said. "Was there a technical failure in the control room or was somebody not watching?"

"But something can happen with the signals, too," added Carmichael, who now works with the University of Denver's Intermodal Transportation Institute.

"The signal system is not foolproof," said retired Burlington Northern-Santa Fe train engineer John Hiatt, who is now a rail-accident investigator with a Minnesota law firm.

NTSB investigators said they were still gathering data to figure out what happened, including why one of the trains failed to pull onto a side track as the other train approached on the main line.

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, who were not related. The conductor who escaped virtually unharmed is Juan Zurita, whom the NTSB planned to interview.

An early review found no problems with the signal system along the tracks near Goodwell and the track appeared normal, said Rosekind, the NTSB official.

The NTSB will check phone records to ensure that workers were not distracted from their duties by cellphones, though Rosekind said no cellphones were found in the wreckage. 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 that the NTSB was checking the crew members' recent work schedules and rest periods, and their evaluations.

The agency 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 trains 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.

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 Highway Patrol. "I would liken it to some of those way overdone Hollywood stunt scenes."