The National Transportation Safety Board has raised concerns about staffing procedures related to the plane crash at LaGuardia Airport in New York that left two pilots dead Sunday night.

The NTSB’s investigation has so far revealed there were two air traffic controllers in the tower at the time an Air Canada plane crashed into a fire truck, and at least one of them was doing several jobs, according to NTSB chair Jennifer Homendy. But Homendy focused on systemic issues, rather than individual failings, at a Tuesday press conference.

“I would caution pointing fingers at controllers and saying distraction was involved. This is a heavy workload environment,” she said.

Here’s what else to know.

The NTSB have flagged their concerns several times

Homendy said it is often standard during the midnight shift for two controllers to carry out the duties of several controllers. But, given LaGuardia’s busy airspace, Homendy questioned the use of the practice there.

“That’s certainly something we will look at as part of this investigation: Would that make sense? Why would that make sense at LaGuardia?” she said.

A local controller and a controller in charge were in the tower at the time of the accident.

The local controller is responsible for managing active runways and the airport’s immediate airspace, while the controller in charge oversees all safety operations. However, the controller in charge was also acting as the clearance delivery controller, who gives pilots permission to depart, Homendy said.

Homendy said the NTSB has conflicting information on whether the local controller or the controller in charge was also serving as the ground controller, who manages vehicle activity on taxiways, Homendy said.

“Certainly I can tell you that our air traffic control team has stated this is a concern for them for years,” she said.

Both controllers were working the overnight shift, Homendy noted.

“The midnight shift, as a reminder, is one that we have, many times at the NTSB, raised concerns about, with respect to fatigue,” Homendy said. “We have no indication that was a factor here but it is a shift that we have been focused on in past investigations.”

During a Monday press conference, Transportation Secretary Sean Duffy said LaGuardia’s air traffic control is relatively well staffed. The airport wants 37 controllers working there. Duffy said Monday there were 33 controllers, with seven more in training.

What happened in the final 3 minutes of the cockpit recording

The NTSB recovered the cockpit voice recorder Monday, and sent it to the NTSB’s lab in Washington, D.C. for analysis. NTSB senior aviation accident investigator Doug Brazy summarized what happened in the last three minutes of the recording.

Brazy said as the plane approached the runway, the flight crew had completed their landing checklist and alerts were sent out that the plane was getting closer to the ground.

After the landing checklist was complete, an unknown airport vehicle called into the control tower, but the audio was “stepped on,” or interrupted, by another transmission, Brazy said.

The tower received a transmission from the firefighters that they wanted to cross the runway. The firefighters were responding to reports of fumes coming from a United Airlines plane. The controllers granted the request, Brazy said.

The tower controllers instructed the truck to stop nine seconds before the recording ended. However, there was no transponder, or radio receiver and transmitter, in the truck, Homendy said.

Tower controllers may use an Airport Surface Detection System, Model X (ASDE-X) to track surface movement of planes. However, the system did not send an alert in this instance, Homendy said.

She read the NTSB tech center’s analysis of the failing at the press conference: “ASDE-X did not generate an alert due to the close proximity of vehicles merging and unmerging near the runway, resulting in the inability to create a track of high confidence.”

Eight seconds before the cockpit recording ends, it sounds like the plane lands, Brazy said. Six seconds out, the first officer transferred control of the plane to the captain. Four seconds out, the tower controllers told the firefighters to stop again.

What NTSB still doesn’t know

Homendy has stressed that while the NTSB has a lot of information, it is preliminary and needs to be verified. Some information may also change, she said.

She said the NTSB still does not know who made the radio transmission that was stepped on, why one of the controllers was still on duty after the crash, or if the firefighters heard the directives to stop. Investigators also do not know whether the pilots saw the truck or if there was any confusion in the cockpit.

“We rarely, if ever, investigate a major accident where it was one failure,” Homendy said. “Our aviation system is incredibly safe because there are multiple, multiple layers of defense built in to prevent an accident. So when something goes wrong, that means many, many things went wrong.”