ARLINGTON, Va. — All V-22 Osprey aircraft were grounded earlier in the week, but most were back in the air by Wednesday, said Mike Welding, a spokesman for the V-22 program.

The Marine Corps and Air Force both use the Osprey, which is a tilt-rotor aircraft that can hover like a helicopter and fly like a regular fixed-wing aircraft.

On Saturday, several loose bolts were found in one MV-22 Osprey deployed to Iraq, Welding said. The bolts were part of a component that controls the proprotors.

Welding stressed the aircraft was on the ground at the time, but Naval Air Systems Command grounded all 84 Marine Corps and Air Force Ospreys on Monday as a precaution.

As of Wednesday, 76 Ospreys had been inspected, of which four had loose bolts — all of which were deployed to Iraq, he said. Two of those four aircraft have been fixed and returned to the air.

The Air Force’s 11 Ospreys have all been inspected, and none was found to have any defects, said Don Arias, a spokesman for Air Force Special Operations Command.

Unlike the Marine Corps, all of the Air Force’s Ospreys are located in the United States, Arias said.

The remaining Ospreys were expected to be inspected by the end of Wednesday, Welding said.

The cause of the problem is under investigation, he said.

The Osprey is meant to replace the Marine Corps’ aging CH-46 Sea Knight helicopters.

Officials are looking at fitting Air Force and Marine Corps Ospreys with a 7.62 mm minigun with a 360 degree field of fire underneath the aircraft.

The weapon system, which would complement the rear machine gun Ospreys now have, would also include a separate turret with cameras that would allow a controller inside the Osprey to see targets.

The minigun option is still being evaluated and officials have not yet determined when to field it, Arias said.

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