Flying helicopters near Ronald Reagan National Airport always carries some risk. But the conditions on the moonless night of Jan. 29, when an Army Black Hawk helicopter and an American Airlines passenger jet collided, were unusually challenging.

Many of the factors that contributed to the disaster are still being uncovered as investigators from the National Transportation Safety Board try to reconstruct the collision that killed 67 people. The midair crash, which caused wreckage from both aircraft to tumble into the icy Potomac River below, was the nation’s deadliest aviation accident since 2009.

Investigators have said the helicopter was flying about 100 feet higher than authorized in its designated portion of the airspace and are trying to determine why.

But interviews with helicopter pilots suggest that the Black Hawk was also dealing with a set of complex flying conditions, some of which are typical for the bustling area around National Airport outside Washington and some of which were unique to the series of events that happened last Wednesday. And the crew was flying an older-model aircraft that lacked certain safety technologies in its cockpit that are commonplace in those of commercial airplanes in the United States.

“Given the complexity of everything going on there, it is a higher-risk place to fly,” said Austin Roth, a former Black Hawk instructor for the Army who says he often flew the helicopter routes near National Airport while in service.

N.T.S.B. safety investigators have not assessed any blame on the Black Hawk crew, which Defense Secretary Pete Hegseth described as “fairly experienced.”

The safety agency said on Tuesday that there was still information that needed to be collected from the helicopter, a process that is expected to begin this week when its wreckage is lifted from the Potomac. Investigators said the two aircraft collided at 300 feet — a detail that has raised questions about how the helicopter got off course, given that it was not authorized to fly higher than 200 feet above ground.

The New York Times, through interviews with six current and former military aviators and a civilian helicopter pilot who frequently flies the routes near National Airport, has pieced together some understanding of the conditions that the crew faced the night of the crash.

The crew in the UH-60 Black Hawk left its home base, Fort Belvoir in Virginia, after dark last Wednesday to conduct a training mission to allow the co-pilot, Capt. Rebecca Lobach, to perform a required annual evaluation flight.

It was part of the small group of military and civilian law enforcement helicopters authorized to fly in the highly restricted airspace over Washington and Northern Virginia. Those pilots must fly along designated routes that generally follow the Potomac and Anacostia Rivers. The air traffic controllers inside the tower at National Airport manage that airspace for helicopters and planes alike.

These routes specify certain altitude restrictions for helicopters along the water, including Route 4, the one that prohibits flying higher than 200 feet over the stretch of the Potomac where the collision occurred.

That restriction, according to several of the pilots, provides little room to maneuver in case of an emergency. At such a low altitude over a river, moving up — not down — is the more realistic response.

Mr. Roth said there are helicopter routes at Dulles International Airport and Baltimore/Washington International Thurgood Marshall Airport that allow pilots to fly over the commercial jet airspace rather than through it, which gives pilots more options in the event of an emergency.

“I can’t think of anywhere where you can fly next to a major airport at 200 feet,” said Mr. Roth, who was in the same unit as the crew of the helicopter that crashed. A combination of dark skies and surrounding city lights — lights that would have been amplified exponentially if the crew members were wearing night-vision goggles — may have distracted them as they searched for nearby air traffic.

“So they’re flying over a black water surface of the Potomac with ground clutter and the buildings behind them,” said Senator Tammy Duckworth, the Illinois Democrat who flew Black Hawk helicopters during her military career.

At about 8:46 p.m. last Wednesday, an air traffic controller warned the helicopter crew that a passenger jet was nearby. That plane, American Airlines Flight 5342, had been redirected from Runway 1, which regional jets commonly used, to the lesser-used Runway 33.

Captain Lobach was most likely in the right-hand seat, said a senior Army official who has flown the National Airport helicopter routes repeatedly but requested anonymity because he was not authorized to speak publicly.

This is significant, the official said, because if the instructor pilot was busy or distracted with something, Captain Lobach’s seat on the right side of the aircraft might have put her in poor position to view the descending American Airlines flight on her left.

Still, other experienced military pilots said they were puzzled at the crash, given that military pilots are trained to be ready for such hazards.

The Black Hawk, a twin-engine aircraft introduced in the 1970s that has inspired a variety of models, has long been a fixture in the U.S. military, both for general purposes and for more tailored missions. In the Army alone, about 2,000 Black Hawks are in operation today.

In the Washington area, which is home to the White House, the Pentagon and several air fields from which both training flights and the transport of the president and other senior officials often originate, Black Hawks are ubiquitous.

The 12th Aviation Battalion at Fort Belvoir flies two types of Black Hawks: the UH-60L, an old model, and the VH-60M, a newer one. The aircraft involved in the crash was the older model. It does not have the ability to let pilots fly on autopilot but it is not considered insufficient for the job, according to the senior Army official.

Regardless, the official said, the crew flying along the Potomac River would not have found autopilot helpful. Low-level flying, he said, requires constant attention to terrain, obstacles and routes.

The Black Hawks, even the older models, are not especially hard to operate, said current and former military aviators. But the congestion around National Airport, one of the country’s busiest public airspaces, requires particular adeptness and a willingness to hang back if necessary to let passenger jets take off or land safely.

“That aircraft was in the wrong place well before they were in the same literal airspace with the CRJ,” said Jon-Claud Nix, a former Marine Corps helicopter pilot, using the abbreviation for the jet that was involved in the collision.

Mr. Nix, who has reviewed the air traffic control recordings and other public details of the crash, added, “They just needed to hold off a little bit to properly identify or locate their correct traffic.”

He said that in the final moments before the crash, the Black Hawk crew was essentially on its own to avoid collision. That is because the crew, according to a recording of the air traffic control audio, had requested what is known as “visual separation,” which under aviation rules means the crew would search out nearby traffic on its own, without assistance from controllers.

And the older Black Hawk model the crew flew last Wednesday most likely did not have certain air-safety systems that are standard among U.S. passenger jets.

For example, it would not have had the Traffic Collision Avoidance System, nicknamed TCAS, which alerts pilots to the fact that their planes are dangerously close to other aircraft and can redirect pilots to quickly climb or descend if a crash seems imminent.

The pilots say one or all of these factors could have contributed to a tragic sequence of events.

“Especially on that route,” Mr. Roth said, “it’s 200 feet which is a low altitude. It’s in proximity to other aircraft. The lighting conditions are tough and there’s just not many places in the world where all of that is happening to anyone all at once.”