The firetruck that collided with an Air Canada jet at LaGuardia Airport late Sunday night was not equipped with a transponder and may not have heard critical instructions from air traffic control seconds before the fatal crash, the National Transportation Safety Board said Tuesday.

The firetruck was responding to a separate call involving a reported bad smell on another aircraft when it crossed paths with the Air Canada jet shortly before midnight. The jet’s two pilots were killed at the scene, and several people onboard were injured.

NTSB Chair Jennifer Homendy said that at the time of the crash, two air traffic controllers were working the midnight shift at LaGuardia, handling multiple responsibilities in the tower.

According to the NTSB’s preliminary timeline, the aircraft’s landing checklist was completed one minute and 20 seconds before the collision. A minute and three seconds before impact, a transmission from the control tower instructing the firetruck to stop was “stepped on” — meaning it was interfered with by another transmission and may not have been received.

Twenty seconds before the crash, air traffic control cleared the firetruck to cross the runway. Nine seconds later, the tower instructed the truck to stop, and the aircraft’s landing gear could be heard deploying. Six seconds before impact, control inputs in the cockpit shifted to the captain. Four seconds before the collision, the tower again told the firetruck to stop, officials said.

Homendy said the sequence of events unfolded “very quickly,” noting that staffing and workload during the midnight shift have been raised as an issue in past investigations.

The NTSB plans to interview the air traffic controllers involved Tuesday afternoon as part of its ongoing investigation.