The Investigators: How and why did Hoboken crash happen?

HOBOKEN (WABC) -- The big mystery as investigators look into the deadly Hoboken train crash is how and why this terrible accident happened. Was it mechanical error or human error? And did the engineer, who by all accounts had a stellar record, simply fail to brake?

Also, as far as whether the crash could have been prevented, many are asking why New Jersey Transit trains don't have positive train control, which brakes on its own if the manual system does not.

The dust and asbestos from the crash debris have significantly slowed access to key evidence, according to one investigator, and one of the key event data recorders and forward-facing video cameras in the damaged train's front cab have yet to be retrieved.

That crucial evidence remains buried under tons of hazardous debris, and investigators can't get to it until OSHA determines it is safe do so. Experts say retrieving the second data recorder is central to the investigation.

"The event recorder, if in good condition, will show us the speed and the braking action," NTSB vice chair Bella Dinh-Zarr said.

While the data recorders will help determine whether any mechanical problems caused the crash, investigators will rely on interviews with the engineer and the two conductors to determine if human factors were involved. It's clear something distracted the 48-year-old engineer, whom sources say for some reason never slowed his train.

"I don't believe, unless I'm corrected, he applied the brakes," said James Sottile, formerly a train accident investigator.

A distracted engineer mistakenly accelerated his Amtrak train near Philadelphia in 2015, causing a derailment that killed eight people. In 2013, an engineer's sleep apnea is believed to have caused a Metro-North derailment that killed four people.

Rail accidents caused by human factors are why Congress mandated positive train control in all passenger trains by 2015, but that deadline has since been pushed back to 2018.

Data from the Federal Railroad Administration website shows NJ Transit has failed to equip a single locomotive or section of track with positive train control. Metro North and Long Island Railroad have made a similar lack of progress, making it unlikely passengers in the New York area will see the safety measure for many years.

"The technology is there," Senator Robert Menendez said. "It should be implemented, and NJ Transit is way behind of all in this regard."

One expert says positive train control does not work well in low speed-restricted areas such as the Hoboken Terminal, but the NTSB cited a lack of PTC in a 2011 PATH station accident as a contributing cause.

Sources say the engineer's cell phone has been retrieved, and the data from that will be downloaded to see if its use was a factor.