SUTTER, Calif. (AP) — A U-2 spy plane’s flight path was routine before something went wrong and the high-flying aircraft slammed into a mountain range in Northern California, the military said. The two pilots ejected, leaving one dead and another injured.

The U.S. Air Force is investigating what caused the plane, assigned to the 1st Reconnaissance Squadron, to go down shortly after taking off from Beale Air Force Base around 9 a.m. PDT Tuesday. Military officials have not released the pilots’ names or any information about the condition of the surviving airman.

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The pilots had ejected from the aircraft before it plunged in the Sutter Buttes, a mountain range about 60 miles (97 kilometers) north of Sacramento.

Officers from Beale Air Force Base and from the Air Force Safety Center, which oversee all inspection and safety functions for the Air Force, will seek to identify “the root cause” of the crash, Keith Wright, a spokesman for the safety center, told The Sacramento Bee.

Col. Larry Broadwell, the base commander, said the flight, including its flight path was routine before the crash. He pledged to support the family of the deceased pilot and said surveillance pilots will mourn the loss.

“These incidents, while extremely tragic and hard for us to overcome, they’re incidents that we do overcome,” Broadwell said. “I am confident that the U-2 squadrons here and the U-2 squadrons around the world are going to come off the mat stronger than they were before.”

The U-2 “Dragon Lady” is a surveillance and reconnaissance plane capable of flying above 70,000 feet (21,336 meters), an extremely high altitude that’s twice as high as a typical commercial airliner flies. The U-2 is known as one of the most difficult aircraft to fly at low altitudes due to the characteristics that allow it to travel near space, according to an Air Force fact sheet.

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Beale Air Force Base is home to the Air Force’s fleet of single-seat U-2s and a double-seat variant used for training pilots to fly the specialized aircraft. It also is the base for the T-38 Talon, a training aircraft, and the RQ-4 Global Hawk, an unmanned surveillance drone. It houses 4,500 military personnel.

“We are saddened by our Airman’s death & offer condolences to the family & all who are mourning this tremendous loss,” Gen. Dave Goldfein, the Air Force chief of staff, said on Twitter.

Ejection seats allow military pilots to get out of a stricken plane and parachute safely to the ground. After the death in this instance, military investigators will look into whether the chute properly deployed and whether the pilot hit debris after ejecting, said Michael Barr, an aviation safety instructor at University of Southern California who flew fighter missions in Vietnam.

“If the chute didn’t properly deploy, that would be fatal,” Barr said.

The U-2 is slated for retirement in 2019 as the military relies increasingly on unmanned aircraft for intelligence gathering, though senior U.S. lawmakers from California are pressuring the Air Force to delay the retirement.

A U-2 based at Beale crashed in 1996 and slammed into the parking lot of a newspaper in Oroville, California. The pilot and a woman who had just renewed her newspaper subscription were killed.

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