DENVER (AP) — The purpose of recent nighttime drone flights over northeast Colorado has remained a mystery to authorities who are trying to learn the identities of the operators.

The drones have flown over Phillips and Yuma counties for the last week, the Denver Post reported Monday.

The Phillips County Sheriff’s Office cannot explain where the drones are coming from or who is flying them.

The Federal Aviation Administration, the Drug Enforcement Administration, the U.S. Air Force, and the U.S. Army Forces Command said they do not have information about the aircraft.

The group of at least 17 drones have estimated wing spans of 6 feet (1.8 meters) and fly between 7 and 10 p.m., Phillips County Sheriff Thomas Elliott said.

The drones remain about 200 to 300 feet (61 to 91 meters) in the air and fly steadily in square patterns of about 25 miles (40 kilometers), Elliott said.

“They’ve been doing a grid search, a grid pattern,” Elliott said. “They fly one square and then they fly another square.”

An FAA spokesman said drone pilots are not required to file flight plans unless they are in controlled airspace such as areas near an airport.

The estimated size and number of drones makes it unlikely they are being flown by hobbyists, Phillips County Undersheriff William Myers said.

Myers watched eight of the drones flying along the Yuma County border Friday, he said.

“Overhead they were probably doing 30, 40 mph,” he said. “They weren’t racing or flying around with speed.”

At the same time, a single drone hovered 25 miles (40 kilometers) away over the town of Paoli, remaining there throughout the night, while eight more drones flew over Haxtun, about 10 miles (16 kilometers) from Paoli, Myers said.

“They do not seem to be malicious,” Elliott said. “They don’t seem to be doing anything that would indicate criminal activity.”

The sheriff’s office has received nine calls about the drones since last week and Elliott said residents no longer need to report sightings.

“We just want to know if one lands, if we can get our hands on it, or if they see someone operating them, that’s what we’re looking for now,” Elliott said. “We know they exist.”