One of only two air traffic controllers on duty at LaGuardia Airport cleared a fire truck to cross a runway just 12 seconds before an Air Canada flight touched down, leaving little time to avoid the collision that killed both pilots, US investigators say.

The National Transportation Safety Board (NTSB) is working to determine which of the airport’s many layers of safety precautions failed and allowed the fire truck onto the runway on Sunday night, local time.

Among the areas being explored are whether the practice of two controllers on duty overnight is enough, why a runway warning system did not alert to the possibility of a crash, and whether the fire truck heard the controller’s last-second pleas to stop.

“We rarely, if ever, investigate a major accident where it was one failure,” NTSB chair Jennifer Homendy said.

“When something goes wrong, that means many, many things went wrong.”

The Canada-based pilots died, and several passengers were injured when the plane, which originated in Montreal and carried more than 70 people, slammed into the fire truck.

Most on board were able to escape the mangled aircraft, and a flight attendant still strapped in her seat survived after being thrown onto the tarmac.

NTSB investigators have not yet interviewed the firefighters, who were also injured, or found whether they braked or turned to avoid the collision, Ms Homendy said.

The regional Air Canada jet collided with a vehicle at LaGuardia Airport in New York. (Reuters: Adam Gray)

Two controllers on duty at time of crash

Investigators want to know more about the role of the air traffic controllers and what they were doing while juggling a late-night emergency involving another plane — a strong odour reported in the cabin of an outbound United Airlines jet.

Ms Homendy warned against jumping to conclusions.

“I would caution against pointing fingers at controllers and saying distraction was involved. This is a heavy workload environment,” she said.

Having two controllers on duty in the control tower was typical for a late-night shift, but had long been a concern for the NTSB, she said. Both were early into the shift when the crash happened.

Audio recordings from the control tower revealed a controller appearing to admit he “messed up” just moments after the crash.

The tower had been busier than expected because flight delays pushed the number of arrivals and departures after 10pm to more than double what was scheduled, according to data from aviation analytics firm Cirium.

Planes were landing every few minutes, with a dozen flights arriving between 11pm and when the crash happened less than 40 minutes later. At the same time, the tower was coordinating the emergency response to the unusual odour, making flight attendants feel ill.

The crash may lead to questions about whether having two controllers on the overnight shift is enough at major airports. That has been the minimum since 2018, after several instances of controllers falling asleep while working solo.

Audio captures moment of deadly Air Canada plane crash at New York airport

Transponder not fitted on fire truck

LaGuardia is one of 35 major US airports with an advanced surface surveillance system to help eliminate dangerous runway incursions and prevent crashes.

Controllers in these airports have a display in the tower that is supposed to show them the location of every plane and vehicle.

The system, known as ASDE-X, did not work as intended this time because the fire truck was not fitted with a transponder, Ms Homendy said.

There were also emergency vehicles behind the fire truck that stopped in time, and the close proximity of the vehicles merging kept the system from triggering an alarm, she said.

More work was needed to determine whether an alert could have prevented the crash, she said.

Asked about the lack of a transponder in the fire truck, the Port Authority of New York and New Jersey, which operates the airport, said it was “unable to comment due to the ongoing investigation”.

The jet was operated by Jazz Aviation, Air Canada’s regional partner. (Reuters: Adam Gray)

Timeline shows final seconds before impact

The NTSB laid out a timeline of the final moments after reviewing the Air Canada jet’s cockpit voice recorder.

Investigators said that 25 seconds before the crash, the fire truck asked to cross the runway where the plane had been cleared to land nearly two minutes earlier.

One controller cleared the truck to cross the runway five seconds later, when the plane was a little more than 30 metres from the ground, the NTSB found.

Then, just nine seconds before the two collided, the tower told the fire truck to stop — a second before the plane’s landing gear touched down.

Ms Homendy said it appeared that the airport’s runway status lights were working, which might have warned the fire truck driver not to cross the runway even if the controller approved it.

The lights embedded in the pavement are designed to automatically turn red when a runway is occupied to signal to vehicle operators and pilots not to enter.

The controller could also see the vehicles and the plane out the tower window, said John Cox, chief executive of Safety Operating Systems, but there may not have been enough time to prevent the crash once the fire truck pulled onto the runway.

The crash came at a time of increasing frustration with air travel in the US, caused by long security lines because of the government shutdown, winter storms and rising costs.