F-35 fighter jets were sent up to investigate after the object was first detected on Thursday, according to a US official. Kirby told reporters that the first fly-by of US fighter aircraft happened Thursday night, and the second happened Friday morning. Both brought back "limited" information about the object. But the pilots later gave differing reports of what they observed, the source briefed on the intelligence said.

Some pilots said the object "interfered with their sensors" on the planes, but not all pilots reported experiencing that.

Some pilots also claimed to have seen no identifiable propulsion on the object, and could not explain how it was staying in the air, despite the object cruising at an altitude of 40,000 feet.

The conflicting eyewitness accounts are partly why the Pentagon has been unable to fully explain what the object is, the source briefed on the matter said.