(Bloomberg) -- A system designed to help air-traffic controllers monitor vehicles and airplanes on the ground failed to alert personnel at LaGuardia Airport before an Air Canada Express jet collided with a fire truck late Sunday evening, killing the two pilots.

An analysis found the system, known as ASDE-X, issued no alert to the two controllers in the tower due to “the close proximity of vehicles merging and unmerging near the runway,” US National Transportation Safety Board Chair Jennifer Homendy said during a press conference on Tuesday.

The emergency response vehicle also wasn’t equipped with a transponder that would have helped pinpoint its location, Homendy said.

The revelations represent new clues for NTSB investigators examining why an Air Canada jet carrying 76 people slammed into a Port Authority of New York and New Jersey fire truck that was crossing an active runway.

The fire truck was responding to a different emergency on a United Airlines Holdings Inc. flight after pilots reported an unusual odor coming from the cabin. The truck, operated by two first responders, requested clearance to cross runway 4, where the Air Canada jet was due to land, and was given the go-ahead by air traffic control.

Though shortly after, a controller could be heard telling the truck driver to stop. Later, what sounds like the same controller said, “I messed up,” when discussing the matter with another pilot.

Homendy told reporters on Tuesday that two controllers were guiding traffic from LaGuardia’s tower late Sunday night. A total of four people were on duty at the time, though only two were in the so-called cab — the panoramic, glass-enclosed room atop the tower where air-traffic controllers work, Bloomberg reported earlier on Tuesday.

Homendy said it’s “standard operating procedure” at the airport to have two controllers in the tower cab for the “midnight shift,” and that they often perform combined duties normally handled by other controllers. One of those controllers signed in at 10:45 p.m. New York time for an eight-hour shift. The other clocked in at 10:30 p.m.

Investigators will continue to examine staffing at the time of the crash as part of their investigation, Homendy said.

Having just two controllers in the cab isn’t uncommon, especially since traffic is typically lighter at that time of day, according to people familiar with the matter. The collision occurred Sunday at about 11:40 p.m. local time.

LaGuardia was busier than expected late Sunday, with bottlenecks stemming from weather delays and long lines at security checkpoints that snaked outside the terminal.

A total of 31 flights were scheduled to depart or arrive from 10 p.m. until 11:37 p.m., just before the crash, according to data compiled by Cirium, an aviation analytics company. However, the actual number during that span was 70.

US Transportation Secretary Sean Duffy said Monday that LaGuardia is a “well-staffed airport” in terms of air-traffic controllers, with about 33 personnel certified to handle flights at the facility against a target of 37.

(Updates with NTSB comments from the first paragraph.)

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