(CNN) A potentially key piece of evidence has been recovered in the hunt for what caused a deadly crash of a commuter train in New Jersey, authorities said Friday afternoon.

Investigators retrieved one of two event recorders from the New Jersey Transit train that hurtled past its stopping point Thursday at the Hoboken station and rammed through a passenger concourse, NTSB Vice Chairman Bella Dinh-Zarr said at a news conference.

Packed with passengers, the train slammed into a bumper block, went airborne and hit the concourse at about 8:45 a.m. during rush hour, killing a woman waiting on the platform and injuring more than 100 others.

The recorder, retrieved from the locomotive in the back of the train, could provide information on the train's speed, use of brakes and throttle position. The NTSB sent the recorder to its manufacturer in Kentucky for downloading, Dinh-Zarr said.

Dinh-Zarr told reporters the NTSB team in New Jersey is on a fact-finding mission and won't come to any conclusions about what caused the crash, she said. That decision will come later from different NTSB officials.

A second event recorder and a forward-facing image recorder, located in the front of the train, have not been recovered yet because of asbestos, structural damage to the train station and other dangers, Investigator in Charge Jim Southworth said.

"Once it is safe, we will go in there. We just want to make absolutely sure. The weather has been a bit of a hindrance," said Dinh-Zarr.

There is no time-sensitive element to recovering the recorder, so investigators will take time recovering it, Dinh-Zarr said.

Investigators have interviewed train crew members, including the engineer, a conductor and a rear brake man, Dinh-Zarr said, declining to reveal what they said. Blood and urine samples were sent to a laboratory for toxicology tests, she said.

Woman killed by debris

Of the 114 people injured, four patients remained hospitalized Friday evening.

Three patients are in stable condition and are expected to be discharged from Jersey City Medical Center in the next 24 hours, hospital spokesman Mark Rabson told CNN. One person is in intensive care in guarded condition, Rabson added.

Fabiola Bittar de Kroon, 34, was killed while waiting on a station platform.

Fabiola Bittar de Kroon, 34, of Hoboken, died after debris struck her while she was on the platform, New Jersey Gov. Chris Christie said.

The crash has indefinitely suspended New Jersey Transit train service to and from the Hoboken station, one of the busiest transit hubs in the New York area, forcing thousands of people to adjust their commuting plans.

New Jersey Transit posted a customer notice on its website advising commuters of their options until the damage can be assessed and repaired.

A separate train system, PATH , continues to run through Hoboken. Hudson-Bergen Light Rail service has been restored into and out of Hoboken and will continue to operate on a full weekday schedule, according to the New Jersey Transit website.

Structural damage appears severe in social images posted after the crash at the Hoboken terminal.

The train was at the end of a 17-stop route that had started more than an hour earlier in Spring Valley, New York. The Hoboken hub primarily serves the Lower Manhattan commuter market.

Images posted on social media showed part of the station's canopy had collapsed. Witnesses described people helping bloodied passengers, some trapped by debris, from the packed front car.

Engineer Thomas Gallagher has been released from the hospital.

The engineer, Thomas Gallagher, 48, was treated and released from a hospital. He has worked 29 years for New Jersey Transit.

A neighbor, Tom Jones, told CNN affiliate WABC-TV that Gallagher said his childhood dream was to become an engineer. "Just a fine, fine family," Jones said of the Gallaghers. "Great next-door neighbors, wonderful kids, very caring people about others."

Speed, 2011 crash, safety systems examined

The investigation will focus in part on speed and why the train didn't stop well before the bumper block, a device meant to halt trains that pass their stops.

Also part of the investigation, according to the NTSB: a train that overran its stop at the same station in 2011 , injuring more than 30 people.

The NTSB also said it will consider whether a safety system called positive train control -- which combines GPS, wireless radio and computers to monitor trains and stop them from colliding, derailing or speeding -- might have prevented Thursday's crash had it been installed.

New Jersey Transit has not yet installed positive train control, although it does use an older safety system. Congress originally required the installation of the newer safety system by the end of 2015 but extended the deadline to the end of 2018.

The NTSB determined the probable cause in the 2011 crash of a PATH train at Hoboken as "the failure of the engineer to control the speed of the train entering the station."

A contributing factor was "the lack of a positive train control system that would have intervened to stop the train and prevent the collision," the agency said.

On Thursday, Christie and New York Gov. Andrew Cuomo cautioned people to wait until investigations are completed before concluding positive train control would have prevented the latest crash.

"That's speculation that can only be based upon the cause of the accident, and until we know the cause of the accident were not going to be able to know what steps we can take in the future to avoid an accident like this," Christie said.

The NTSB already is reviewing video from surveillance cameras that were along the train's path, Dinh-Zarr said.

'Next thing I know, I'm on the floor'

Bhagyesh Shah, who rode in the front car on his way to work, said the train didn't appear to slow as it entered the station.

"The next thing I know, I'm on the floor. We are plowing through something ... and when the train came to a stop, I could see the parts of the roof on the first car and some of the debris next to me," Shah said.

A New Jersey Transit worker at the station said he heard an explosion-like sound as the lead car, coming into the station fast, slammed into the bumper block.

"It went up and over the bumper block, through the depot ... and came to rest at the wall by the waiting room," worker Mike Larson said.

"It was going considerably faster than it should have normally been."