ARLINGTON, Va. (AP) — A jet with 60 passengers and four crew members collided with an Army helicopter Wednesday while approaching the Ronald Reagan National Airport near Washington. The aircraft plummeted into the Potomac River and multiple people were killed.

The crash occurred in some of the most tightly controlled and monitored airspace in the world, just over 3 miles (5 kilometers) south of the White House and the U.S. Capitol.

Here are some things to know about the collision:

The crash

The midair crash happened around 9 p.m. when a regional jet at the end of a flight from Wichita, Kansas, collided with a military helicopter on a training exercise, according to the Federal Aviation Administration.

A few minutes before the jet was to land, air traffic controllers asked American Airlines Flight 5342 if it could land on a shorter runway and the pilots agreed. Controllers cleared the jet to land and flight tracking sites showed the plane adjust its approach to the new runway.

Less than 30 seconds before the crash, an air traffic controller asked a helicopter if it had the arriving plane in sight. The controller made another radio call to the helicopter moments later: “PAT 25 pass behind the CRJ.” There was no reply. Seconds after that, the two aircraft collided.

The plane’s radio transponder stopped transmitting about 2,400 feet short of the runway, roughly over the middle of the Potomac.

Emergency response

Authorities were conducting a massive search-and-rescue operation. Inflatable boats were launched into the river and first responders set up light towers from the shore to illuminate the area. Helicopters from law enforcement agencies across the region also flew over the scene in a methodical search for bodies and survivors.

Fatalities

Sen. Roger Marshall of Kansas hinted that the number killed will be high. “When one person dies it’s a tragedy, but when many, many, many people die it’s an unbearable sorrow,” he said.

Airport

Located along the Potomac River, just southwest of Washington, Reagan National is a popular choice because it’s much closer to the city than the larger Dulles International Airport.

All takeoffs and landings from Reagan Airport were halted. It will reopen at 11 a.m. Thursday, the Federal Aviation Administration announced.

Investigation

Investigators will try to piece together the aircraft’s final moments before their collision, including contact with air traffic controllers as well as a loss of altitude by the passenger jet.

Aircraft involved

The helicopter was a UH-60 Blackhawk based at Fort Belvoir in Virginia, according to the U.S. Army. A crew of three soldiers were on board the helicopter, an Army official said.

The other aircraft was a Canadian-made Bombardier CRJ-701 twin-engine jet and was manufactured in 2004. It can be configured to carry up to 70 passengers.

History of fatal aircraft crashes

Fatal crashes of commercial aircraft in the U.S. have become a rarity. The last was in 2009 near Buffalo, New York. All 45 passengers and the four crew members were killed when the Bombardier DHC-8 propeller plane crashed into a house. One person on the ground also was killed.

The incident Wednesday recalled the crash of an Air Florida flight that plummeted into the Potomac on January 13, 1982, killing 78 people. That crash was attributed to bad weather.