The Air Force has indefinitely grounded all Boeing Co. F-15 fighter jets after a plane flown by the Missouri Air National Guard crashed last week during a training exercise.

"The cause of that accident is still under investigation, however, preliminary findings indicate that a possible structural failure of the aircraft may have occurred," the Air Force said in a statement released late Sunday.

Maj. Cristin Marposon, an Air Force spokeswoman, said Monday that the country's fleet of 676 F-15s, including mission critical jets, were grounded Nov. 3 for "airworthiness concerns."

While the F-15 is grounded, the Air Force said it would rely on Lockheed Martin Corp.'s F-16 fighter jet, and other aircraft, for routine missions. However, the F-15 will remain available for combat or other emergency situations, Marposon said. F-15s were scrambled in domestic airspace on Sept. 11, 2001.

The Air Force said it will produce an investigative report on Friday's crash near Salem, Mo., within 60 days.

The grounding of an aircraft is not common, Air Force spokeswoman Jennifer Bentley said Monday, adding that she was unsure when the service last took this step.

"It's not a decision we made lightly," Bentley said. "Until we can find out what the problem is, it's the safest thing to do."

Boeing spokeswoman Patricia Frost said the company was cooperating with the Air Force, but could not provide additional comment due to the pending investigation.

Col. Robert Leeker, commander of the 131st Fighter Wing, said Friday the plane had been among four planes split into pairs and were engaging in one-on-one training flights in which speeds of 400 to 450 mph are typical. The other planes returned safely.

A 10-year veteran of the guard whose name and rank were not released safely ejected from the aircraft when it crashed in Dent County, Mo. The pilot, who suffered a dislocated shoulder, a broken arm and minor cuts and bruises, was released Saturday from the hospital. The one-seat plane, a 1980 model worth $40 million, was assigned to the 131st Fighter Wing, an Air National Guard base at Lambert-St. Louis International Airport.

This is not the first time an F-15 has crashed this year. In May, the fighter jet crashed just outside of Vincennes, in southwestern Indiana during a flight practice with four F-16 jets from the Indiana Air National Guard's 181st Fighter Wing, based in Terre Haute. In June, another F-15 jet from the 142nd Fighter Wing, Oregon Air National Guard based in Oregon, went down in the Pacific Ocean during a training mission. And in a separate incident in June, an F-15 crashed near Eielson Air Force Base in Alaska.

Marposon said the decision by the Air Force to ground the F-15s was based solely on the accident in Missouri.

F-15s are located in bases throughout the United States including Alaska and Hawaii, as well as Japan and the Middle East.

The F-15 was originally manufactured by St. Louis-based McDonnell-Douglas, which was purchased by the Chicago-based Boeing Co. about a decade ago. Boeing delivered its last F-15 to the U.S. Air Force in December 2004, but still manufacturers the aircraft for other customers, according to the company's spokeswoman.

The Air Force is currently seeking to replace its aging F-15s, the oldest dating back to 1975, with Lockheed Martin Corp.'s F-22 Raptor.

Shares of Boeing added 8 cents to $98.05 in extended trading, after rising 21 cents to close at $97.97.