A Dec. 21, 2019, Facebook post by the Phillips County Sheriff's Office said that officials were aware of and have been investigating the multiple reports of drone sightings in the county over the last week.

The drones appear after dark, flying in formation over swaths of land in rural Colorado and Nebraska. For weeks, they have dominated headlines in local newspapers, fueled intense speculation on social media and unsettled residents, who have flooded law enforcement with calls.

So far, the aircraft remain a mystery. Officials in multiple counties say they have not been able to determine who is flying them or why. The Federal Aviation Administration is now investigating, an agency spokesman told The Washington Post on Thursday.

In the absence of information, wild theories abound in the small communities where the drones have been spotted, encompassing everything from government surveillance to alien activity. Others offered less-nefarious explanations, suggesting a private company is using them to map or survey land, or perhaps practicing for drone shows.

"There are many theories about what is going on, but at this point, that's all they are," Sheriff Todd Combs of Yuma County, Colorado, wrote in a Facebook post. "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."

The drones, described by the Denver Post as having 6-foot wingspans and numbering at least 17, showed up in mid-December in northeastern Colorado. They emerge nightly around 7 p.m., flying in squares of about 25 miles and staying about 200 feet in the air, the newspaper reported. By about 10 p.m., they're gone.

Local authorities say the mysterious visitors do not appear to be malicious and may not be breaking any laws. Combs noted in his post that they are operating in airspace controlled by the federal government and, as far as he could tell, abiding with federal regulations.

Yet the unexplained aircraft, buzzing above homes nightly, have still caused alarm — so much so that officials with multiple sheriff's departments have cautioned residents against shooting them down.

"I have been made aware of several comments about shooting down a drone," Morgan County, Colorado, Sheriff Dave Martin said in a statement. "I ask that you NOT do this as it is a federal crime."

Wyatt Harmon and his girlfriend, Chelsea Arnold, chased a cluster of drones after they flew over his property in the Colorado county of Washington. The couple tailed them for 15 miles, hitting speeds above 70 miles per hour, according to NBC's "Today," which featured an interview with the two on Tuesday.

Harmon said during the interview that the aircraft could descend and take off "very fast."

"It's kind of just scary," Arnold added. "It's more unnerving than anything."

Another Colorado resident, Haley Harms, told Denver's 9 News she is organizing a team of "drone watchers" to monitor the activity. She hopes that mapping the drone appearances might help answer the question of what they're doing and where they might turn up next.

Groups devoted to tracking the drones have also popped up on Facebook.

"When you put on the landscape blinking lights and fleets of things doing patterns over my fields, that doesn't make me comfortable at all," said Harms, who lives in an area so remote she can hear trucks miles away. "It's curious that no one seems to know why or who or what."

The FAA is considering a rule requiring most drones to be identifiable. Spokesman Ian Gregor told the Times the timing of the proposed regulation, which was announced Dec. 26, was coincidental. It would allow drones to be tracked remotely, which would provide "critical information to law enforcement and other officials charged with ensuring public safety," the agency said in a summary of the proposal.

In his Facebook post, Combs wrote that drone technology was outpacing the passage of rules, describing the existing ones as "very vague" and calling for new "common sense" regulation. He said he was "as frustrated as you are" by the lack of answers.

"These drones have made residents in our community very nervous and anxious," he wrote. "People do not like the unknown as it upsets the balance of our lives."