If the controller involved in the Air Canada crash was performing both air and ground duties, that would be inconsistent ⁠with the LaGuardia tower’s standard operating procedures.

Air traffic controller staffing at LaGuardia ​airport on the night an Air Canada jet collided with a fire truck may have violated the facility’s procedures by combining roles before midnight, ‌according to a document seen by Reuters.

The crash at the New York airport at about 11:37 p.m. ET on March 22, which killed both pilots, has revived concerns over U.S. air traffic control staffing shortages and the workload of controllers across the country.

Staffing shortages, including at the supervisor level, are placing controllers into combined roles handling local air and ground traffic more often, according to several air traffic ​controllers across the country.