The NTSB is still trying to determine who was performing the duties of the ground controller, who manages all aircraft and vehicle movement on taxiways.

Investigators continue to probe for answers in Sunday’s deadly collision at LaGuardia Airport, as officials examine why a runway safety system failed to issue an alert before a Port Authority fire truck crashed into a landing passenger airplane.

Jennifer Homendy, chairwoman of the National Transportation Safety Board, said at a news conference Tuesday that the airport uses a safety system called ASDE-X to track surface movements of aircraft and vehicles.

“ASDE-X did not generate an alert due to the close proximity of vehicles merging and unmerging near the runway, resulting in the inability to create a track of high confidence,” Homendy read from an analysis of the system’s performance.

She also said the fire truck, which was responding to a separate emergency involving a United Airlines flight, did not have a transponder. Homendy could not say whether it is common for ground vehicles to have transponders, but noted it “would have been helpful.”

Jeff Guzzetti, an NBC News aviation analyst, said the truck having a transponder “very well could have provided that alert that the controller could have used.”

Air Canada Express Plane Collides With Fire Truck At LaGuardia AirportOfficials investigate after an Air Canada Express plane collided with a fire truck on the tarmac at LaGuardia Airport.Spencer Platt / Getty Images

The two pilots on the Canada Air Express plane were killed in the collision, and more than 40 others were taken to the hospital. Passengers have praised the pilots, Antoine Forest and Mackenzie Gunther, for saving their lives.

“I think everybody on that plane feels very grateful that they’re all alive, and they all pretty much give it to the pilots,” said Jack Cabot, who was returning home from a spring break ski trip.

Cabot, 22, told NBC News he could feel the pilots trying to slow the plane before the crash. He suffered a bruised cheek and a sore neck when his head struck the seat in front of him.

Christopher Pal, a professor in Montreal who was coming to New York for meetings, recalled the brakes being “a little louder than normal.”

“It happened very suddenly, and then the plane went a little bit left and right, I think, and then it stopped,” he said. “Everybody was just in shock.”

Pal said he went through the emergency exit onto the wing and slid the short distance to the ground. He encouraged others to follow him.

“I just said, ‘Look, jump into my arms. I’ll catch you if you need help.’ So I caught some people, and I just stayed there until it looked like most people were out.”

Pal said he only suffered minor injuries.

According to the plane’s cockpit voice recorder, the truck was cleared to cross the runway 20 seconds before the crash. Nine seconds before the collision, the air traffic control tower ordered the truck to stop, Doug Brazy, the NTSB’s senior aviation investigator, said at Tuesday’s news conference. At the 4-second mark, the tower issued a second stop order.

Photos of the wreckage showed the plane’s nose had been torn off, with mangled pieces dangling toward the ground.

According to Homendy, two controllers were on duty in the tower the night of the crash: a local controller, who manages active runways and the immediate surrounding airspace, and the controller in charge, who oversees all safety operations. The controller in charge was also performing the duties of the clearance delivery controller, which provides pilots with their departure clearance, she said.

The NTSB is still trying to determine who was performing the duties of the ground controller, who manages all aircraft and vehicle movement on taxiways.

While Homendy said it is common practice across national airspace to have two controllers during the midnight shift, the NTSB has raised concern about the practice in the past.

“The midnight shift, as a reminder, is one that we have many times at the NTSB raised concerns about with respect to fatigue,” she said. “Again, I do not know, we have no indication that was a factor here, but it is a shift that we have been focused on in past investigations.”

Stephen Abraham, a retired air traffic controller with 28 years in the industry, said he understands the agency’s concern, but stressed that the ongoing staffing shortage of controllers remains a problem.

“You can’t invent or make people,” he said in an interview Wednesday. “If you put three people” on the midnight shift, he said, “it means you have less people during the day and at night when it’s busier. There’s just not enough people.”

Abraham said it could be close to a year before investigators have a full picture of what actually happened. He believes the NTSB will ultimately identify “a litany of mitigating factors” that contributed to the collision, including the weather and the emergency aboard the United flight.

On the night of the crash, the fire truck was responding to a report of an odor on a United plane after flight attendants started to feel ill before takeoff. Bryan Bedford with the Federal Aviation Administration said at a news conference Monday that the airport was experiencing mist, fog, moderate winds, and roughly 4 miles of visibility in rainy conditions.

“The whole aviation system is designed so if one piece of the equation fails, it still works,” Abraham said. “There’s technological equipment to prevent these things. There’s human beings to prevent these things, and then there are human factors, elements to prevent these things. How many of them didn’t work to cause this to happen?”