DENVER ― They mostly come at night. Mostly.

Swarms of drones have been spotted flying across eastern Colorado and western Nebraska since mid-December, typically between the hours of sunset and midnight.

They often fly in groups and travel along seemingly coordinated, grid-like paths. On the night of Dec. 20, the Phillips County Sheriff’s Office, along with its counterparts in neighboring Yuma County, tracked more than 16 such drones.

Colorado Gov. Jared Polis (D) pledged Thursday to “get to the bottom” of the mystery flights and ensure they aren’t putting people or property at risk. “It’s certain they have caused a bit of a stir,” he said in a statement.

Polis said he has asked the Department of Public Safety to investigate, but that he hasn’t yet received “any substantial information” beyond publicly available reports.

That dovetails with just about every other government agency that might be behind the flights. They all told The Denver Post they don’t know who’s responsible. That list includes the Federal Aviation Administration, the U.S. Air Force, the Defense Advanced Research Projects Agency, the Department of Defense, the Environmental Protection Agency, the North American Aerospace Defense Command, the Drug Enforcement Administration, U.S. Army Forces Command, Fort Carson, Intel, Amazon and the University of Colorado, Boulder.

Private drone companies operating in the area have also denied any responsibility.

RJ Sangosti/MediaNews Group/The Denver Post via Getty Images via Getty Images Lincoln County Sheriff's Deputy Justin Allen stands outside of his squad car looking at the night sky near Limon, Colorado, on Jan. 2.

FAA regulations require certified and commercial drone pilots to register their drones, maintain line-of-sight on the drone while in flight, fly at or below 400 feet, fly only during the daytime and never over third parties, and prohibit pilots from operating more than one drone at a time.

Witness reports suggest the drones in question are likely violating at least some of those rules.

Jennifer Rollins, who was visiting rural Yuma County over the holidays, captured video footage of one of the mystery drones in the night sky and provided it to the Post. The distant object was one of several hovering in the sky at the time, shifting up and down with red and white lights flashing. It moved out of sight after about half an hour.

“You could see that it was rotating,” she said. “You could see things sticking off to the sides. It was crazy.”

The FAA can and does grant waivers, but without knowing who’s behind the flights (and if they’re a recipient of a waiver), it’s impossible to know if they’re acting lawfully.

“Most people are very reasonable, and they say it could be somebody mapping or doing topography,” Michael Yowell, a sheriff’s captain in Lincoln County, told The New York Times. “But you can’t rule out what you don’t know.”

Yowell estimated 30 drones are out flying every night.

Washington County Sheriff Jon Stivers told 9News that his office has received reports some of the drones are larger than 6 feet in diameter.

RJ Sangosti/MediaNews Group/The Denver Post via Getty Images Michael Yowell of the Lincoln County Sheriff's office shows a map of the county that marks confirmed sightings of mysterious drones in the area.

Morgan County Sheriff Dave Martin, meanwhile, is urging residents not to commit a felony in an attempt to shoot a drone down. (In 2013, the city of Deer Trail considered issuing﻿ drone-hunting permits, but ultimately rejected the notion after a less-than-enthusiastic response from the FAA.)

“I have been made aware of several comments about shooting down a drone,” Martin said. “I ask that you NOT do this as it is a federal crime. Please be patient and let our investigative efforts try to figure this out.”

Multiple sheriffs offices and police departments in the affected areas are scheduled to hold a closed-door meeting with federal agencies on Monday, the Sedgwick County Sheriff’s Office said on Facebook.

“There are many theories about what is going on, but at this point, that’s all they are,” Yuma County Sheriff Todd Combs said Tuesday in a statement. “I think we are all feeling a little bit vulnerable due to the intrusion of our privacy that we enjoy in our rural community, but I don’t have a solution or know of one right now.”