ARLINGTON, Va. (AP) — Police boats combed the banks of the Potomac River on Friday, slowly scanning the shoreline in the rain as investigators sought clues into the midair collision that killed 67 people and raised questions about air traffic safety around the nation’s capital.

The black box from the Army Black Hawk helicopter that collided with a commercial jetliner and crashed into the Potomac River has been recovered, investigators announced. They are reviewing that flight data recorder along with two recovered earlier from the jet.

No one survived the Wednesday night collision. The remains of 41 people had been pulled from the river as of Friday afternoon, including 28 that had been positively identified, Washington, D.C., Fire Chief John Donnelly Sr. said at a news conference. He said next of kin notifications had been made to 18 families, and he expects all the remains to be recovered.

The wreckage of the plane’s fuselage will probably have to be pulled from the water to get all the bodies, he said.

“This is heartbreaking work,” Donnelly said, noting that more than 300 responders were taking part in the effort at any one time, including teams of divers and two U.S. Coast Guard cutters, at least one of which carries a crane. “It’s been a tough response for a lot of our people.”

It was unclear how long the recovery operation would take.

“We’re working as fast as we can,” Donnelly said. “We need your patience.”

The American Airlines jet was carrying 60 passengers and four crew members. The helicopter had three soldiers on board.

Although Ronald Reagan National Airport reopened, two of its three runways remained closed to keep aircraft from flying over the crash scene, said Terry Liercke, the airport’s vice president and manager. Roughly 100 flights were canceled Friday.

The Federal Aviation Administration also heavily restricted helicopter traffic around the airport, the Department of Transportation said in a statement, hours after President Donald Trump claimed in a social media post that the Army Black Hawk had been flying higher than allowed.

Investigators have recovered the cockpit voice recorder and flight data recorder of the American Airlines jetliner, which collided with the chopper as the plane was coming in for a landing at the airport, which is just across the Potomac from Washington.

The flight data recorder was in good condition and its information was expected to be downloaded shortly, National Transportation Safety Board member Todd Inman told reporters. He said water entered the cockpit voice recorder, and while that’s not unusual, it adds to investigators’ work.

The helicopter’s data, contained in a single black box, was at NTSB headquarters and appears to be undamanged, Inman said.

Investigators are examining the actions of the military pilot as well as air traffic control, after the helicopter apparently flew into the jet’s path. NTSB investigations normally take at least a year, though investigators hope to have a preliminary report within 30 days.

Military aircraft frequently conduct such flights in and around the capital to practice routes they would fly if key government officials had to be quickly moved during a major catastrophe or attack.

Other potential factors in the crash, including the helicopter’s altitude and whether the crew was using night vision goggles, are still under investigation, Defense Secretary Pete Hegseth told Fox News Channel.

On Friday the Army released the names of two of the soldiers who died: Staff Sgt. Ryan Austin O’Hara, 28, of Lilburn, Georgia, the crew chief; and Chief Warrant Officer 2 Andrew Loyd Eaves, 39, of Great Mills, Maryland. Eaves was one of the pilots on the helicopter.

But in an unusual move, the name of the third soldier is not being released for now at the family’s request, the Army said.

The FAA on Friday barred helicopters from flying over a roughly 6-mile (10-kilometer) stretch of the Potomac and parts of its shoreline, including over the airport, according to the Department of Transportation statement. The new limits exempt helicopters on emergency medical flights, active law enforcement and air defense missions, and carrying the president.