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The jet crashed off base on Opportunity Way in Riverside County, about 50 miles east of Los Angeles, at about 3:45 p.m., base officials told NBC News. Hazardous materials crews were securing potential hazards, including ordnance that may have been on the aircraft, military and state officials said.
Aerial video of the scene showed a broad hole in the roof of a large warehouse-like building bearing the logo of See Water Inc., which manufactures pumps, controls and other equipment for the water and wastewater industries. Authorities were examining a white and orange parachute along the roadside, in an industrial area of warehouses and large businesses near Interstate 215.
The interstate remained closed in both directions Thursday night.
Timothy Holliday, the base's deputy fire chief, said that five people on the ground were treated for injuries, two of them at hospitals and two at the scene. Information on the fifth person wasn't immediately available, along with the patients' conditions, but Holliday described all of their injuries as non-life-threatening.
Capt. Fernando Herrera, a spokesman for the Riverside Unit of the California Department of Forestry and Fire Protection, said later that 12 people underwent decontamination procedures for possible hazardous materials and were treated at hospitals for minor injuries, primarily exposure to debris.
It couldn't immediately be determined whether the 12 people cited by Herrera included the six people cited by Holliday.
Preliminary information indicated that the jet developed hydraulic problems as the pilot was trying to land, Holliday said. The pilot, who wasn't publicly identified, declared an emergency and then ejected when he couldn't gain control of the jet, Holliday said.
A witness, Cameron Lee, told NBC Los Angeles that he was on the freeway when he saw the jet, which began "tilting from side to side."
"As soon as I saw it begin tilting, the pilot ejected," Lee said. "He went off to the right [and] the plane went over to the left."
"I couldn't believe what I was seeing," said Lee, who said at least 10 fire trucks were at the scene almost immediately.
Herrera said the warehouse building is equipped with sprinklers, which contained any flames. But because it wasn't immediately known what ordnance the jet may have been carrying, most of the businesses in the facility were evacuated as rescue and investigative teams took their time reviewing the scene.
"Anything that is in there may be of concern not only to us but to the military," he said.
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