The National Transportation Safety Board has confirmed that LaGuardia Airport’s surveillance system, used to track the surface movement of aircraft and ground vehicles on the runway, did not generate an alert at the time of Sunday night’s plane crash due to the proximity of vehicles merging and unmerging.

The airport’s ASDE-X alert system is the tool that did not work as intended the night of the incident, NTSB Chairwoman Jennifer Homendy told reporters at a Tuesday afternoon press conference. The system is used to help reduce runway collisions, according to the Federal Aviation Administration.