Warning bells were not required and were not in place at a railroad crossing where a Metro-North commuter train slammed into a sport utility vehicle on the tracks Tuesday, federal investigators said Friday as they continued to investigate the fiery crash that killed six people.

Robert Sumwalt, a member of the National Transportation Safety Board, said warning bells are only required at railroad crossings that include a pedestrian walkway, and the crossing where the accident occurred did not have a walkway. He said investigators will do tests to determine whether the driver of the SUV could have heard the train’s horn and whether warning bells at the crossing could have helped.

The crossing gate and warning lights functioned properly before the 6:30 p.m. crash in Valhalla, a suburb north of New York City, investigators have said. But a 2009 plan to install a third set of flashing lights 100 to 200 feet up the road to give motorists coming around the bend a few seconds' extra warning was never carried out, The Associated Press reported.

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The train’s engineer sounded a standard series of horn blasts as he approached the crossing, Sumwalt said. After spotting the SUV on the tracks, the engineer hit the emergency brake and sounded the horn for the four more seconds before the crash, Sumwalt said.

"He sounded the horn all the way up to the point of collision," Sumwalt said.

The train slowed from 58 mph to 49 mph in the four seconds between the activation of the emergency brake and the crash, Sumwalt said. The driver of the 2011 Mercedes ML350 and five passengers on the train were killed.

The engineer told investigators he saw sparks after striking the vehicle, and the compartment filled with smoke. He emerged to find the first train car on fire, and helped several passengers to safety and carried another one off the train, Sumwalt said.

As the train pushed the SUV about 650 feet up the track, the electrified third rail was torn up, impaled the vehicle and was sent crashing into the first train car. Twelve sections of the rail — measuring 39 feet each — piled up inside the train car, Sumwalt said.

Investigators are looking at whether the track was de-energized as it was designed when it started to break apart. U.S. Sen. Richard Blumenthal on Friday called for improvements to train crossings. "In a country that can put a man on the moon we can design better grade crossings that save lives," he said.

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12 39-foot sections of third rail penetrated the first car of the train during the accident sequence. #MetroNorth — NTSB (@NTSB) February 6, 2015

Train involved in Valhalla, NY, #MetroNorth accident at maintenance facility to be further examined. pic.twitter.com/Prq4r9VTG4 — NTSB (@NTSB) February 5, 2015

— Phil Helsel