Shortly before midnight on Sunday, Air Canada Express Flight 8646, a Canadair CRJ-900 operated by Jazz Aviation LP, collided with a firetruck after landing at New York’s La Guardia Airport.

Publicly available audio of air-traffic-control radio communications with planes and ground equipment, as well as visual playback of automatically reported aircraft positions and speed, make it possible to determine the exact sequence of events that led to the crash.

10:23 p.m.: The Air Canada Express flight, bearing the flight number AC8646 and the air-traffic call sign “Jazz 646,” takes off from Montreal carrying 72 passengers and four crew members.

10:40 p.m.: United Airlines 2384, a 737 MAX headed for Chicago, is taking off from La Guardia Runway 13 when the flight crew decides to abort the takeoff due to a warning light. It taxis back to the end of the runway and tries again 18 minutes later, then aborts the takeoff a second time at 11:19. The plane, which at this point is on the eastern side of the airport, then taxis a short distance away from the runway and reports that an odor in the cabin has caused flight attendants to feel ill. A fire truck is dispatched from the airport fire station, located on the western side of the airport.

11:33 p.m.: The Air Canada flight, descending through 3,000 feet over Bay Ridge, Brooklyn, turns left and lines up to land on Runway 04 at La Guardia. A light rain is falling, and a light wind is blowing from the northeast.

11:35 p.m: La Guardia tower grants the Air Canada flight permission to land on Runway 04 behind a Southwest 737 MAX arriving from Denver.

It’s a busy time; the airport is operating at maximum capacity, with multiple aircraft lined up to land and takeoff, and others taxiing to and from the two active runways.

11:36:54 p.m.: The fire truck is at the Delta intersection of runway 4 and needs to cross it from west to east in order to reach the United flight.

The tower asks, “Who’s the vehicle needing to cross the runway?” “Truck 1 and company,” comes the reply.

11:37:07 p.m.: The tower gives the truck permission to cross the active runway: “Truck 1 and company, cross 04 at Delta.”

11:37:10 p.m.: The fire truck responds, “Truck 1 and company crossing 04 at Delta.”

At this moment the landing plane is passing over the arrival end of the runway at an altitude of 50 feet and traveling at a speed of 134 knots, or 154 mph. It is approximately a half-mile from the fire truck.

11:37:13 p.m.: The tower controller turns his attention to Frontier 4195, an Airbus 320 bound for Miami, that at the moment is taxiing toward Runway 04: “Frontier 4195, just stop there, please.”

11:37:17 p.m.: Apparently suddenly recognizing the dangerous situation he has created on runway 04, the controller urgently calls over the radio to the firetruck: “Stop, stop, stop, stop, Truck 1! Stop, stop, stop. Stop, Truck 1, stop stop. Truck 1, stop!”

On the ATC audio recording, an alarm is audible.

11:37:20 p.m.: The Air Canada Express plane has touched down and is traveling at 144 mph as it passes taxiway Charlie, 2,000 feet from the approach end of the runway.

Taxiway Delta lies 700 feet ahead, a distance that takes just three seconds to cross at that speed.

The firetruck has pulled out onto the runway.

At this point the Air Canada flight has few options. It is in a high-drag landing configuration, losing speed and energy quickly. Trying to lift off again to hop over the truck would be difficult or impossible. Constrained by the narrow width of the runway, it can’t swerve around the fire truck.

Then there is the simple lack of time. According to an FAA circular published in 2016, it takes pilots on average 12.5 seconds to recognize and respond to an imminent collision.

11:37:23 p.m.: The CRJ impacts the firetruck head on. The front of the aircraft, including the cockpit, is crushed, killing the captain and first officer instantly. The impact flips the firetruck onto its side, injuring two of the firemen inside.

According to the New York Post, a flight attendant survived the impact after being ejected from the wreckage while still strapped into her jumpseat.

The plane veers off to the right of the runway as it comes to a stop.

11:37:46 p.m.: The tower controller urgently tells the next plane landing on Runway 04, a Delta 737 inbound from Detroit, to abort its landing and climb away from the airport. He then calls the Air Canada Express flight crew, not knowing that they are dead: “Jazz 646, Jazz 646, I see you collided with a vehicle there, just hold position, I know you can’t move, vehicles are responding to you now.”

11:55:38 p.m.: The tower controller radios to another pilot and confesses that he bears responsibility for the collision: “I tried to reach out when I stopped, were dealing with an emergency earlier and I messed up.”

In the aftermath of the crash, the FAA closed La Guardia to all inbound and outbound air traffic until 2 p.m. Monday. By Monday morning, more than 300 flights had been canceled or delayed.

The crash was the second fatal commercial air crash in two years, coming 14 months after the collision last January between a regional jet and a U.S. Army helicopter near Reagan National airport in Washington, D.C. Prior to that crash, there hadn’t been a fatal airline crash in the United States in more than 15 years.

It’s not yet clear how many flight controllers were on duty at the time of the accident, but ATC staffing in U.S. control towers has been an ongoing issue for years and was cited as one of the contributing factors in the Reagan accident.

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