(The Hill) – Most reported UFO sightings are likely the result of foreign spying or airborne trash, governmental officials said, according to The New York Times.

An intelligence report last year revealed that the U.S. government had encountered more than 140 unidentified aerial phenomena, commonly known as UFOs, since 2004 and could not confidently explain most of those incidents.

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The report led to a flurry of interest in UFOs and was followed by the first congressional hearing on the issue in more than 50 years.

However, U.S. officials have solved many of these supposed mysteries, the Times reported on Friday. Most of the recent unidentified aerial phenomena can be attributed to trash in the sky or foreign surveillance activity, such as by drones believed to be connected to China, officials told the outlet.

The Pentagon has kept most of its conclusions on foreign surveillance classified, so as not to reveal to China and other countries that it is aware of their activities, the officials said, according to the Times. They told the outlet that Congress has been briefed on some of the conclusions.

Some older incidents also remain officially unexplained simply because there is not enough data to make a final conclusion, the Times also noted.