On Tuesday, the National Transportation Safety Board provided new details of the final minutes before an Air Canada jet collided with a fire truck at LaGuardia Airport in New York.

The timeline from federal investigators and air traffic audio reviewed by The New York Times both suggest that the controllers may have been distracted before the crash, which killed the plane’s two pilots and left dozens injured late Sunday.

Here are critical moments leading up to the deadliest collision at the airport in more than three decades:

Air traffic controllers were responding to an emergency with United Airlines Flight 2384 several minutes before the crash, posing a possible distraction to air traffic controllers.

After being on the tarmac for over two hours, the United flight, bound for Chicago, had aborted its first takeoff attempt at 10:40 p.m. Passengers were told the plane had “a transient issue,” according to a passenger who requested anonymity in order to protect her privacy.

The pilots made a second attempt at takeoff about 40 minutes later and aborted again.

At 11:31 p.m., United flight had declared an emergency and requested a gate assignment, according to air traffic control audio reviewed by The Times. An odor on the plane had sickened members of the flight crew.

Four minutes later, the plane was assigned a gate and told to wait for emergency responders.

Air Canada Express Flight 8486 was set to land at LaGuardia Airport when the approach controller, who manages flights as they near the airport, ordered the airplane to contact the control tower, National Transportation Safety Board officials said on Tuesday.

The flight crew began lowering the landing gear. The plane was cleared to land on Runway 4 and advised that it was No. 2 for landing, said Doug Brazy, a senior aviation accident investigator with the National Transportation Safety Board.

One minute and 26 seconds before the crash, an electronic callout indicated that the plane was 1,000 feet from the ground.

A passenger told The Times that a flight attendant warned the passengers to leave any luggage behind if the plane made an emergency landing. It’s unclear why this warning was made.

Around 11:37 p.m., or 25 seconds before the crash, “Truck 1” made a request to cross Runway 4 at Taxiway D, the same runway that the Air Canada jet was set to land on. The request was made to respond to the emergency with the United Airlines plane.

Five seconds later, the truck, which later crashed with the jet, was cleared to enter the runway, officials said. An air traffic controller quickly responded: “Truck 1 and company, cross 4 at Delta.”

The officers aboard “Truck 1” read back the runway clearance. That’s a mandatory practice to ensure that the message was received correctly, and to verify that both the air traffic controllers and the recipient of the information understood the instructions.

Five seconds later, the plane was 30 feet above the ground, and the tower instructed a Frontier Airlines aircraft to hold its position.

LaGuardia Airport has a “Runway Status Lights” system that includes red runway entrance lights at taxiway and runway crossings. The lights, which are set in the pavement, activate automatically when high-speed traffic is on the runway or approaching it.

While there is speculation about whether the fire truck ran a red runway status light, a Times analysis of the crash footage suggests the lights on Runway 4 appeared to be functioning properly when the fire truck entered the runway.

By design, the lights can go dark a couple of seconds before a landing or taking-off plane passes the intersection. The truck may have entered the runway in that brief window. What remains unknown is whether the crew members heard the controller’s instruction to stop, and, if so, why they proceeded regardless. The lights do not replace clearances given by the air traffic controllers.

Nine seconds before the collision, an air traffic controller instructed “Truck 1” to stop. There were other vehicles behind the fire truck that did not proceed to the runway.

“Stop, stop, stop, stop, Truck 1, stop, stop, stop,” the controller said. Sounds consistent with the plane’s landing gear slamming against the pavement could be heard in the audio from the cockpit voice recorder.

Four seconds before the regional jet plowed into the fire truck, the controller again said, “Stop, Truck 1, stop!”

Investigators have not determined whether the operators of the fire truck heard orders to stop before colliding with the Air Canada flight.

The National Transportation Safety Board has raised concerns about staffing procedures related to the plane crash at LaGuardia Airport in New York that left two pilots dead Sunday night.

The NTSB’s investigation has so far revealed there were two air traffic controllers in the tower at the time an Air Canada plane crashed into a fire truck, and at least one of them was doing several jobs, according to NTSB chair Jennifer Homendy. But Homendy focused on systemic issues, rather than individual failings, at a Tuesday press conference.

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

Here’s what else to know.

The NTSB have flagged their concerns several times

Homendy said it is often standard during the midnight shift for two controllers to carry out the duties of several controllers. But, given LaGuardia’s busy airspace, Homendy questioned the use of the practice there.

“That’s certainly something we will look at as part of this investigation: Would that make sense? Why would that make sense at LaGuardia?” she said.

A local controller and a controller in charge were in the tower at the time of the accident.

The local controller is responsible for managing active runways and the airport’s immediate airspace, while the controller in charge oversees all safety operations. However, the controller in charge was also acting as the clearance delivery controller, who gives pilots permission to depart, Homendy said.

Homendy said the NTSB has conflicting information on whether the local controller or the controller in charge was also serving as the ground controller, who manages vehicle activity on taxiways, Homendy said.

“Certainly I can tell you that our air traffic control team has stated this is a concern for them for years,” she said.

Both controllers were working the overnight shift, Homendy noted.

“The midnight shift, as a reminder, is one that we have, many times at the NTSB, raised concerns about, with respect to fatigue,” Homendy said. “We have no indication that was a factor here but it is a shift that we have been focused on in past investigations.”

During a Monday press conference, Transportation Secretary Sean Duffy said LaGuardia’s air traffic control is relatively well staffed. The airport wants 37 controllers working there. Duffy said Monday there were 33 controllers, with seven more in training.

What happened in the final 3 minutes of the cockpit recording

The NTSB recovered the cockpit voice recorder Monday, and sent it to the NTSB’s lab in Washington, D.C. for analysis. NTSB senior aviation accident investigator Doug Brazy summarized what happened in the last three minutes of the recording.

Brazy said as the plane approached the runway, the flight crew had completed their landing checklist and alerts were sent out that the plane was getting closer to the ground.

After the landing checklist was complete, an unknown airport vehicle called into the control tower, but the audio was “stepped on,” or interrupted, by another transmission, Brazy said.

The tower received a transmission from the firefighters that they wanted to cross the runway. The firefighters were responding to reports of fumes coming from a United Airlines plane. The controllers granted the request, Brazy said.

The tower controllers instructed the truck to stop nine seconds before the recording ended. However, there was no transponder, or radio receiver and transmitter, in the truck, Homendy said.

Tower controllers may use an Airport Surface Detection System, Model X (ASDE-X) to track surface movement of planes. However, the system did not send an alert in this instance, Homendy said.

She read the NTSB tech center’s analysis of the failing at the press conference: “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.”

Eight seconds before the cockpit recording ends, it sounds like the plane lands, Brazy said. Six seconds out, the first officer transferred control of the plane to the captain. Four seconds out, the tower controllers told the firefighters to stop again.

What NTSB still doesn’t know

Homendy has stressed that while the NTSB has a lot of information, it is preliminary and needs to be verified. Some information may also change, she said.

She said the NTSB still does not know who made the radio transmission that was stepped on, why one of the controllers was still on duty after the crash, or if the firefighters heard the directives to stop. Investigators also do not know whether the pilots saw the truck or if there was any confusion in the cockpit.

“We rarely, if ever, investigate a major accident where it was one failure,” Homendy said. “Our aviation system is incredibly safe because there are multiple, multiple layers of defense built in to prevent an accident. So when something goes wrong, that means many, many things went wrong.”