The U.S. Air Force has returned nearly 700 F-15 fighter jets to service after a Nov. 2 crash grounded the fleet.

Although the pilot was not killed, the military ordered the fleet of planes, made by Boeing Co., grounded a day later on "airworthiness concerns."

After completing safety inspections on more than 500 of the F-15s, the Air Force said Wednesday the fleet could be returned to service.

Industry experts and the Air Force are working with a federal plane crash investigation board to determine what caused the plane to go down, said Air Force Combat Gen. John Corley in a Nov. 21 memo to F-15 pilots.

There is one squadron of F-15s at Langley Air Force Base in Hampton.

The plane, flown by the Missouri Air National Guard, went down during a training exercise near Salem City. Corley asked pilots to "remain vigilant" to mitigate any unknown risks as the fleet returns to service.

The Air Force seeks to replace aging F-15s, some more than 30 years old, with Lockheed Martin Corp.'s F-22 Raptor. The latest version of the F-15 is being used in operations in Iraq and Afghanistan. Non-combat but critical-mission flights were flown on Lockheed's F-16s while the F-15 fleet was grounded.

The F-15 was first manufactured by St. Louis-based McDonnell-Douglas, which was purchased by Chicago-based Boeing some 10 years ago. Boeing delivered its last military F-15 to the Air Force in late 2004 but still manufactures the aircraft for non-military customers, the company said.

Shares of Boeing added $2.13, or 2.4 percent, to close at $89.54 on Friday's holiday-shortened session.