US Navy veterans involved in the 2004 Nimitz UFO sightings have told a US website "unknown" individuals turned up after the event and made them turn over all recordings and data.

The encounter has since been confirmed by the US Defense Department.

In the video above: US Navy confirms UFO videos.

In 2017 it released footage from a Navy FA-18 Super Hornet showing a mysterious flying object during the 2004 event.

However, it is understood much more footage and data remains classified, its location unknown.

The incident occurred in November 2004, about 160km off the California coast when a number of unidentified flying objects were encountered by fighter pilots from the Nimitz Carrier Strike Group.

They had been sent to investigate radar signals that did not match those given off by any known aircraft and which the ship's tracking systems had been following for several days.

Now a trio of navy veterans - not pilots but other officers - involved in that incident have told US website Popular Mechanics what happened as it unfolded and in the aftermath.

The men were sailing on a training mission as part of the US Navy's Strike Carrier Group 11.

System technician Gary Voorhis said he was helping bed down new radar systems and helping get the kinks out aboard the USS Princeton.

Concerned about the readings he was getting, Voorhis recalibrated the new technology.

"Once we finished all the recalibration and brought it back up, the tracks were actually sharper and clearer,” Voorhis told the site.

"Their radar cross-sections didn’t match any known aircraft; they were 100 per cent red. No squawk, no IFF (Identification Friend or Foe)."

"I couldn’t make out details, but they'd just be hovering there, then all of a sudden, in an instant, they’d dart off to another direction and stop again,” Voorhis said.

"At night, they’d give off a kind of a phosphorus glow and were a little easier to see than in the day."

Some time after all this data was recorded, Voorhis claims two people arrived on the ship via helicopter.

They were never identified.

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And barely 20 minutes after that, Voorhis's senior officers told him to turn over all the recordings and delete any that remained on the ship.

"They even told me to erase everything that's in the shop — even the blank tapes," he told Popular Mechanics.

Meanwhile, aviation officer Patrick Hughes was on the nearby USS Nimitz.

It was his job to secure hard drive data recorders from the airborne early-warning aircraft - the recorders that had stored data about the encounter.

He too was visited by his commander accompanied by two unknown individuals and told to turn over the hard drives.

"They were not on the ship earlier, and I didn’t see them come on. I’m not sure how they got there," Hughes said

"We put them in the bags, he took them, then he and the two anonymous officers left,” Hughes said.

And also on the USS Princeton, Ryan Weiglet, a helicopter power plant specialist, told the site the two men had arrived on his ship, flown off and returned "with a bunch of bags".

He told Popular Mechanics the pair repaired to the Admiral's Quarters and a guard was stationed outside the door.

Another witness - who did not wish to be named - told a similar story to the site.

"What really made this incident alarming was when a Blackhawk helicopter landed on our ship and took all our information from the top-secret rooms,” the witness said.

"We were all pretty shocked and it was an unspoken rule not to talk about it because we had secret clearances and didn’t want to jeopardise our careers."