The crash occurred over the Potomac River near Reagan National Airport, killing 67 people.

It's one of the most restricted airspaces in the country, but this particular area has long-worried pilots like retired Army officer Darrell Feller.

That's probably the most dangerous spot on all those routes in D.C.

So here's everything we know about why that area is so precarious.

The 33-mile radius around Washington, D.C. is considered a special flight rules area.

Pilots have to have advanced clearance from the FAA to fly here, and aircraft must have altitude-encoding transponders, among other things.

Within that, there's a 17-mile flight-restricted zone around Reagan Airport, with even stricter rules.

And then there are prohibited areas around the White House, National Mall, and the Vice President's residence.

But even with the restrictions, it's still one of the most congested airspaces in the country, with civilian, military, police, and other government aircraft constantly operating.

To help manage this, D.C. has designated helicopter corridors, as well as altitude restrictions, to keep everyone safe.

So what happened during the crash?

Flight data shows the Black Hawk helicopter flying south down a helicopter corridor over the Potomac River.

Military officials say the pilots were experienced and were conducting an annual night training mission known as a checkride.

Meanwhile, American Airlines Flight 5342 was traveling north toward Reagan National.

It was originally supposed to land on Runway 1, with a flight path that goes up the river.

But at approximately 8.42 p.m., minutes before landing, the jet was told to use a different, shorter runway.

Runway changes aren't unusual, but it meant the plane had to bow right before turning sharply left and cutting across the helicopter corridor, a move that pilots say makes the jet hard for helicopters to see, especially at a low altitude.

And you can't see them if they're moving toward you, away from you, or you can't see them at all.

There are also questions about staffing of air traffic control that night and whether more people should have been working.

Another potential problem was altitude.

Helicopters are supposed to fly below 200 feet in this part of the corridor.

But flight data shows the helicopter may have risen above 300 feet as the airplane was descending from 400 feet.

Officials say data from the aircraft's recording device still needs to be investigated to confirm.

But Feller says even if altitude was a factor, something else likely went wrong before that.

A professional Army aviator is not going to fly within 100 feet of a passenger jet.

He's just not going to do that.

I would suspect that he didn't see the jet.

During a press briefing, Defense Secretary Pete Hegseth said this.

There was some sort of an elevation issue that we have immediately begun investigating at the DOD and Army level.

Army CID is on the ground investigating.

Top-tier aviation assets inside the DOD are investigating, sir, to get to the bottom of it so that it does not happen again because it's absolutely unacceptable.

After the crash, the FAA announced it would restrict helicopter traffic in the area where the Black Hawk was traveling.

For more UN videos visit www.un.org