Two senior military observers say they watched a mysterious black triangular object flying at an extraordinarily high speed over a sensitive site in the western US earlier this year, in the latest tranche of reported UFO sightings released on Friday by the Trump administration.

The unidentified object showed none of the characteristics of a conventional military aircraft, their report to the defense department said, and it vanished suddenly and silently within seconds, leaving no exhaust trail or other evidence of its existence.

The claimed sighting, by anonymous sources with “extensive operational experience working alongside government agencies”, occurred at a location that was redacted in the file made public on Friday.

The “unidentified anomalous phenomenon” (UAP), a term that has superseded UFO in military, governmental and scientific circles, resembled a delta-wing drone with a wingspan between nine and 15ft (2.75 to 4.5m), the observers recalled. They calculated its speed at 1,800mph.

Similar to hundreds of other reports released by the Pentagon since May as part of Donald Trump’s promise to bring transparency to decades of previously classified or secret documents relating to UFOs, many other details are vague, inconclusive or blacked out altogether.

While many previous accounts have been accompanied by real photographs or video footage of apparent sightings, the “black triangles” report features nine identical images of the same unidentified canyon with tiny, trilateral silhouettes crudely added to replicate the movement of the object the observers said they saw.

“This does indeed look like little more than a black triangle shape superimposed on a landscape background to illustrate the report. I don’t think it really adds anything substantive to the report itself,” said Greg Eghigian, professor of history and bioethics at Penn State University, and author of the 2024 book After the Flying Saucers Came: A Global History of the UFO Phenomenon.

After an early flurry, the Pentagon’s releases in the second Trump administration have become less frequent in recent months, and noticeably thinner since the US president declared in February that he was ordering full disclosure about the government’s knowledge of UFOs and the search for alien life beyond our planet.

The move followed allegations aired in Congress in 2023 by a former intelligence official that successive administrations engaged in a cover-up, and that the government had possession of crashed alien spacecraft.

Friday’s release contained fewer than 25 text documents, and no videos or images.