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WASHINGTON - As the new decade dawns in the U.S., authorities are trying to unravel a high-tech mystery: why are unidentified drones being repeatedly flown over a portion of the country's heartland at night and who owns them?

For the moment, no one has claimed responsibility for the unmanned aircraft being flown in precise formations over the prairie grasslands of rural northeastern Colorado and southwest  Nebraska.

The Federal Aviation Administration says it has launched an investigation of the mysterious drone sightings, with local residents and law enforcement officials reporting they have watched as the unmanned aircraft, some with wingspans of two meters, have hovered overhead. The aircraft have generally stayed about 45 to 60 meters away from buildings and people.

Drones fly in square grid pattern

One local sheriff's office in Colorado said it tracked 16 drones in late December, their lights blinking as they were flown in a square grid pattern every night from 5 to 10 p.m.

"We believe that the drones, though startling, are not malicious in nature," the sheriff's office tried to reassure residents, while not solving the mystery.

Some residents have speculated that the drones could be mapping the region for possible oil and natural gas drilling exploration.

Event called "unnerving"

One resident, Wyatt Harman, told NBC that seeing the drones — and not knowing why they were flying overhead — was "unnerving."

"They can sit there and hover," he said. "They can descend very fast. They can take off very fast."

"It's creepy," another local resident, Missy Blackman, told the New York Times. "I have a lot of questions of why and what are they, and nobody seems to have any answers."