(AP) - Federal safety investigators haven't been able to determine why the driver of an SUV drove her car into a railroad crossing and into the path of an oncoming train, causing a crash that killed six people outside New York City in 2015.

The National Transportation Safety Board met Tuesday in Washington, D.C., to detail the results of a two-and-a-half year investigation into the crash at a crossing in the town of Valhalla.

The impact sparked an explosion, and flames blasted into the passenger area, burning out the first car of the train. The driver of the SUV and five people aboard the train were killed. More than a dozen others were injured.

NTSB investigators found that the SUV's driver, Ellen Brody, wasn't on the phone, impaired or fatigued. Brody drove onto the tracks and when the gate arm came down onto her SUV, she got out and inspected the vehicle before getting back in and driving further onto the tracks.

Chairman Robert Sumwalt said her actions were the great mystery of the crash.

"There are numerous possibilities which may have contributed to why she got out of the car, why she did not realize the train was approaching," he said. "We examined every possible situation and circumstance and we could not arrive at a definite conclusion."

He said it was impossible to know what was in Brody's mind, but he hypothesized that she had been inching along in traffic, watching the car in front of her, and wasn't aware that the she had driven into a railroad crossing.

"I don't think she realized where she was," he said. "I think it was just a loss of situational awareness."

Meanwhile, the engineer noticed something on the tracks and pulled the emergency brake when he noticed the SUV in the path.

Investigators found all the signals were working properly; the brakes worked and were pulled on time; the warning signs at the crossing worked and were properly marked; the train wasn't speeding; the engineer wasn't fatigued or distracted; the track wasn't faulty; and the emergency exit windows worked.

But the design of the third rail providing power to the train also played a role, they said. They said the rail stayed in one piece, like a 340-foot-long spear, rather than break apart when it was ripped from the ground. The rail then sliced through a passenger car on the train, contributing to the death toll.

NTSB investigators said the lack of a controlled failure mechanism in third rail systems was a potential safety problem. It recommended that railways that use third rails evaluate the safety risks at grade crossings.

The Associated Press, relying on information from an official briefed on the NTSB's findings, reported Monday that investigators had also raised concerns with the unusual third rail design in which power is transferred to the train via a metal shoe riding beneath the powered rail. NTSB officials said at the Washington briefing, however, that they had found no issues with that particular system.

NTSB investigators recommended risk assessments also be conducted for grade crossings. The town of Mount Pleasant where the crash occurred was weighing whether to close the crossing altogether.

But also, drivers must always be aware on railroad crossings, they said.

"There is a lesson drivers must learn, a lesson that has tragically been taught time and time again. The next train is always coming. ... Do not get trapped on the tracks," Sumwalt said.