The drivers of two commuter trains that slammed into New York City-area stations in separate incidents were both suffering from severe sleep apnoea, US investigators have found.
The pair have no memory of the crashes, according to investigative reports and interview transcripts made public on Thursday.
The National Transportation Safety Board said the common circumstances of the September 29 2016, New Jersey Transit crash in Hoboken, New Jersey, and the January 4 2017, Long Island Rail Road crash in Brooklyn warranted combining findings and recommendations in a single investigative report to be released early next year.
People with the disorder are repeatedly awakened and robbed of rest as their airway closes and their breathing stops, leading to dangerous daytime drowsiness.
Sleep apnoea also went undiagnosed in the engineer of a Metro-North commuter train that sped into a 30 mph curve at 82 mph and crashed in New York City in 2013, killing four people.
NJ Transit driver Thomas Gallagher told investigators he only remembered looking at his watch and the speedometer, blowing the horn and ringing the bell before his packed rush-hour train slammed into Hoboken Terminal at more than double the 10mph speed limit.
A conductor standing on a platform told investigators he could not see the driver through the cab window as the train rumbled into the station, indicating Mr Gallagher may have slumped down or fallen.
Falling debris from the impact killed a woman standing on a platform. About 110 people aboard the train were hurt.
"The next thing I remember was a loud bang," Mr Gallagher recalled, according to a transcript of his October 1 2016, interview.
"I was getting hit with dust and dirt. I was thrown about the cab.
"I hit my head, the back of my head, I presume on the wall behind me. And then I had a period where I was going in and out of consciousness."
LIRR driver Michael Bakalo told investigators he only remembered approaching Atlantic Terminal in Brooklyn and then getting thrown from his seat.
He said he was not aware of the impending crash.
More than 100 people were hurt when the train crashed into a bumping post at the end of the tracks.
The impact launched the lead car into the air and it came to rest on top of the concrete platform.
"At that point I didn't know what the hell was going on," Mr Bakalo said.
"I remember being thrown from the seat because I was into the dashboard area, and just, you know, screaming and smoke, and people were laying on the floor in front of me."
The documents released by the NTSB on the two crashes do not come to any conclusions on what caused them but offer a glimpse into what investigators have learned so far.
The NTSB has scheduled a hearing on the crashes for February 6.
The documents released on Thursday indicate Mr Bakalo claimed to have lost "situational awareness", and the train came into the station too fast.
Neither driver had been diagnosed with sleep apnoea before the crashes, according to the documents.
In August, US federal regulators abandoned plans to require sleep apnoea screening for train engineers and lorry drivers, arguing it should be up to railways and haulage companies to decide whether to test employees.