A plane sent to search for drones in northeast Colorado came up empty Monday night, even as a report of a drone coming too close to a medical helicopter on Wednesday night raised concerns.

The plane flew for almost five hours as part of a joint operation Monday in an attempt to identify what witnesses have reported to be large drones flying during the night in northeast Colorado and Nebraska since mid-December, according to the Colorado Department of Public Safety.

The plane crew worked with law enforcement officers on the ground to respond to every report of drone activity that came in during that five-hour span, Micki Trost, spokeswoman for the Colorado Division of Homeland Security and Emergency Management, said in a statement Wednesday.

“During the flight of the [plane], every report of a suspicious drone was investigated and was resolved, and none was substantiated,” she said in the statement.

The plane, which can detect heat signatures, did not “detect any suspicious heat signatures or drones related to the drone reportings,” the statement said.

The agency said Thursday afternoon that it would “activate resources this week” to continue investigations in response to a report of a drone flying dangerously close to a medical helicopter. It also released an information sheet about how to identify suspicious drone activity.

The joint operation with the plane followed the establishment of a task force Monday morning during a meeting of about 75 people from a wide variety of state, local and federal agencies. Members of the group agreed to share information as they investigated the drones.

Authorities say they still don’t know who is flying the drones, and after Monday’s meeting some began to question whether witnesses were actually seeing drones or might be seeing other flying objects such as planes.

A litany of organizations have denied ownership of the drones, including several private drone companies, several gas and oil companies, the U.S. Air Force, the U.S. Department of Defense, the North American Aerospace Defense Command, the Defense Advanced Research Projects Agency, U.S. Army Forces Command, Fort Carson, Intel, Amazon and the U.S. Geological Survey, among others.

Authorities have said the drone operators likely aren’t breaking any laws. Drone pilots are only required to file flight plans with the Federal Aviation Administration if they plan to fly in restricted airspace, like near an airport.