The cannon fire could explain the fact that several civilians were injured by bullets fired during the mission near the opposition stronghold of Benghazi.
Four Harriers were sent from the USS Kearsarge in the early hours of Tuesday morning to provide cover for a pair of Osprey helicopters whose task was to pick up a pilot who ejected from his F-15E Strike Eagle because of a mechanical failure.
Two of the Harriers dropped two 500lb bombs on a convoy of Libyan vehicles that they judged were a threat to the downed pilot, who was hiding in farmland after landing by parachute.
But they also carried out "strafing runs", according to a US military source familiar with the operation.
That may explain accounts given by villagers that when they went to help the pilot they came under fire, with an estimated eight people injured, according to eye-witnesses and medical hospital staff.
It was originally claimed in British media reports that the gunfire must have come from Marines on board the Osprey helicopter that rescued the pilot but US officials have categorically denied that, saying the Marines did not fire any rounds.
Capt Becky Massey, 31, one of the four Osprey pilots, said it was a straightforward operation and that none of the 15 Marines on board her aircraft fired any shots.
"We had had comms with the pilot. When we found him he was in pretty good shape," said Capt Massey, who has flown Ospreys – a hybrid aircraft that can convert from a plane to a helicopter and back again - for four years.
A male pilot who asked not to be named said: "We didn't take any fire and we didn't put down any fire. The only living things we saw in the area (apart from the pilot) were farm animals." The second F-15E crew member, a weapons officer, also ejected and parachuted to the ground but was recovered by Libyan rebels, who later passed him onto the Americans.
The circumstances of how he was transferred into American hands are a mystery, but he was on Wednesday reported to be recovering from his ordeal at Aviano air base in north-eastern Italy.
