U.S. Central Command, which oversees the Middle East, said the crash in western Iraq on Thursday followed an unspecified incident involving two aircraft in “friendly airspace” and that the other plane landed safely.

The crash brings the U.S. death toll in Operation Epic Fury to at least 13 service members, with the seven others killed in combat. About 140 U.S. service members have been injured, including eight severely, the Pentagon said earlier this week.

The KC-135 has been in service for more than 60 years and has been involved in several fatal accidents, most recently in 2013. Adding to concerns about their reliability, the aircraft don’t always carry parachutes.

Here’s what is known so far about the tanker, which is the fourth U.S. military aircraft publicly acknowledged to have crashed since the war against Iran began on Feb. 28:

Cause of crash not immediately known

U.S. Central Command said the circumstances of the crash are under investigation but that the loss of the aircraft was “not due to hostile or friendly fire.”

A U.S. official, who spoke on condition of anonymity to discuss the developing situation, said the other plane involved was also a KC-135. Yechiel Leiter, the Israeli ambassador to the U.S., wrote on X that the other plane landed safely in Israel.

Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, told reporters at the Pentagon on Friday morning that the crash occurred “over friendly territory in western Iraq, while the crew was on a combat mission” and reiterated that hostile or friendly fire was not the cause.

Speaking at the same news conference, Defense Secretary Pete Hegseth called the crew heroes.

“War is hell. War is chaos,” Hegseth said. “And as we saw yesterday with the tragic crash of our KC-135 tanker, bad things can happen. American heroes, all of them.”

Hegseth and Caine spoke to reporters before the deaths of the six crew member had been made public.

Yang Uk, a security expert at South Korea’s Asan Institute for Policy Studies, said it would be rare for a refueling tanker to be downed by enemy fire because such operations are usually conducted in the rear of combat zones.

Last week, three U.S. F-15E fighter jets were mistakenly downed by friendly Kuwaiti fire. All six crew members ejected safely.

The KC-135 is a long-serving tanker plane

The KC-135 Stratotanker is a U.S. Air Force aircraft used to refuel other planes in midair, allowing them to travel longer distances and maintain operations longer without landing. The plane is also used to transport wounded personnel during medical evacuations or conduct surveillance missions, according to military experts.

“The last of these planes were produced in the 1960s,” Yang said.

Based on the same design as the Boeing 707 passenger plane, the KC-135 is set to be gradually phased out as more of the next-generation KC-46A Pegasus tankers enter service.

According to the Congressional Research Service, the Air Force last year had 376 KC-135s, including 151 on active duty, 163 in the Air National Guard and 62 in the Air Force Reserve.

A basic KC-135 crew consists of three people: a pilot, co-pilot and boom operator. Nurses and medical technicians are added in aeromedical evacuation missions.

Refueling typically happens at the back of the plane, where the boom operator is located. A fuel boom is lowered to connect with fighters, bombers or other aircraft. On many of the planes, the boom operator works lying face down while looking out of a window on the underside of the plane.

Some KC-135s can also refuel planes from pods on their wings. The tankers have room to carry cargo or passengers if needed.

Refueling tankers could play an increasingly important role if the Iran war drags on, as U.S. aircraft may need to fly longer missions to pursue Iranian forces retreating deeper into the country, said Yang.

A question about parachutes

KC-135s have been involved in several fatal accidents. The most recent occurred on May 3, 2013, when one crashed after takeoff south of Chaldovar, Kyrgyzstan, while supporting the war in Afghanistan.

In that crash, the crew experienced problems with the plane’s rudder, according to a U.S. Air Force investigation. While the crew struggled to stabilize the plane, the tail section broke away and the plane exploded midair, killing all three onboard.

The most serious mid-air collision involving the plane happened in 1966, when a B-52 bomber carrying nuclear bombs struck a tanker near Palomares, Spain.

The accident caused the tanker to crash, killing four onboard. The disaster led to an extensive decontamination effort to clean up nuclear material dispersed when conventional explosives in the hydrogen bombs detonated after hitting the ground.

The plane has a good safety record overall, is well-maintained and has been updated often with new equipment, said Alan Diehl, a former investigator for the Air Force Safety Center who examined mishaps that involved KC-135s.

But Diehl said an important question is whether this KC-135 was carrying any parachutes. The one that crashed in Kyrgyzstan was not, according to the investigation.

Diehl said the reasoning for not always requiring parachutes, at least in the 1980s and 1990s, included the expense of maintaining them and training to use them. He said K-135s are designed with an escape hatch on the flight deck and a spoiler to help airmen jump clear of the fuselage.

A 2008 news release from an air refueling unit said the Air Force was pulling parachutes from KC-135s, noting that it was statistically safer to stay with the aircraft, “especially when flying over enemy territory.”

“Removing parachutes from military aircraft may sound peculiar, but KC-135s are not like other aircraft,” the news release stated. “They seldom have mishaps, and the likelihood a KC-135 crew member would ever need to use a parachute is extremely low.”

Diehl stressed that it’s unclear whether parachutes would have helped the crew over Iraq. But he said the second plane landing safety suggests the collision may not have been catastrophic.

When asked if the plane that crashed had parachutes, the military would say only that the cause of the incident was still under investigation.

As for why the KC-135 that crashed had six people on board, Diehl said some could have been back-up crew, given that the aircraft can stay in the air for many hours.