Air Force investigators were trying to determine the cause of an apparent collision of two fighter jets that killed one pilot during a training exercise.

The single-seat F-15C Eagles crashed Wednesday into the Gulf of Mexico, said Col. Todd Harmer, commander of the 33rd Fighter Wing, 58th Fighter Squadron. The pilots had ejected and were rescued, but one died later.

The base has suffered a “great loss,” Harmer said in an e-mailed statement. “We will continue to do everything we can to assist our families and airmen at this tragic time.”

The condition of the surviving pilot and the names of both pilots were not released.

The cause of the collision about 35 miles south of Tyndall Air Force Base in the Florida Panhandle was not immediately known, Harmer said. Weather in the area was clear.

The training exercise emphasized “basic maneuvers and tactics,” he said.

A Coast Guard rescue jet located one pilot and radioed the location to a fishing vessel, which picked him up, said Coast Guard Petty Officer James Harless. A Coast Guard helicopter then hoisted the pilot off the vessel.

That pilot told rescuers he saw the other pilot eject but lost him in the clouds, Harless said. He told them the approximate location for the second pilot, who was found by a Coast Guard helicopter.

Both pilots had been with the 33rd Fighter Wing “for quite some time,” Harmer said.

No debris from the jets has been found.

The Air Force grounded all of its F-15s — nearly 700 — after the catastrophic failure of an F-15C during a routine training flight in Missouri in November. The pilot safely ejected.

Most were back in service by January, but others were grounded indefinitely after defects were found.

The Air Force began using the F-15C in 1979. The planes, built by McDonnell Douglas Corp., were deployed to the Persian Gulf in 1991 in support of Operation Desert Storm and have since been used in Iraq, Turkey and Bosnia.

The planes can fly as high as 65,000 feet, and each costs about $30 million, according to the Air Force.