On a recent evening near a secure U.S. facility, three federal agents saw a flying object they couldn't identify: a large "matte black triangle" that moved as fast as a fighter jet, made no sound and then disappeared, according to FBI and Defense Department reports.

Why it matters: The 15-foot flying triangle is classified as a UFO-like "Unidentified Anomalous Phenomena" because the U.S. doesn't have the technology to make such a craft — and neither do its adversaries.

The reports of the drone-like aircraft is of growing interest and concern to intelligence and military officials.

Driving the news: The two reports about the mysterious triangle were included in a batch of 10 documents released Friday by U.S.…