However, the Pentagon's effort to investigate anomalous, unidentified objects -- whether they are in space, the skies or even underwater -- led to hundreds of new reports that are now being investigated, they say.

But so far they have seen nothing that indicates intelligent alien life.

"I have not seen anything in those holdings to date that would suggest that there has been an alien visitation, an alien crash or anything like that," said Ronald Moultrie, under secretary of defense for intelligence and security.

Sean Kirkpatrick, director of the Pentagon's newly formed All-domain Anomaly Resolution Office (AARO), did not rule out the possibility of extraterrestrial life and said he was taking a scientific approach to the research.

"I would just say that we are structuring our analysis to be very thorough and rigorous. We will go through it all," Kirkpatrick said, speaking at the first news conference since AARO was established in July.

"And as a physicist, I have to adhere to the scientific method, and I will follow that data and science wherever it goes."

AARO's mission focuses on unexplained activity around military installations, restricted airspace and "other areas of interest" and is aimed at helping identify possible threats to the safety of U.S. military operations and to national security.

A government report last year documented more than 140 cases of what the U.S. military officially calls "unidentified aerial phenomena," or UAPs, observed since 2004.

All but one of the listed sightings - an instance attributed to a large, deflating balloon - remain unexplained, subject to further analysis, the report said.

For the other 143 cases, the report found that too little data exists to conclude whether they represent some exotic aerial system developed either by a U.S. government or commercial entity, or by a foreign power such as China or Russia.

The 2021 report included some UAPs revealed in previously released Pentagon video of enigmatic objects exhibiting speed and maneuverability exceeding known aviation technology and lacking any visible means of propulsion or flight-control surfaces.

Kirkpatrick said several hundred more cases have been documented since then. The exact figure will be disclosed soon, but a senior Navy official said in May the total number of reported cases had already reached 400.

Congress focused on the new Pentagon push in its annual defense policy bill, which it passed this week. The legislation, which has not yet been signed by President Joe Biden, calls for the Pentagon to prepare a report looking at the historical record of the U.S. government related to UFOs, or unidentified flying objects, going back to 1945.

"That is going to be quite a research project," Kirkpatrick said, acknowledging that Congress sought to ensure that AARO researches all records -- even ones so highly classified that few people know about them.

The Air Force conducted a previous investigation called Project Blue Book, ended in 1969, that compiled a list of 12,618 sightings, 701 of which involved objects that officially remained "unidentified."

In 1994, the Air Force said it had completed a study to locate records relating to the 1947 "Roswell incident" in New Mexico. It said materials recovered near Roswell were consistent with a crashed balloon, the military's long-standing explanation, and that no records indicated that there had been the recovery of alien bodies or extraterrestrial materials.