(CNN) A drone was in "dangerous proximity" to a medical helicopter in Colorado, prompting authorities to step up their investigation into the mysterious drones seen in the state in recent weeks.

The incident, which took place on Wednesday, has increased concern about the suspicious drones, Colorado Emergency Management tweeted on Thursday.

The pilot of a Flight for Life helicopter reported that a drone was flying approximately 100 feet below the aircraft, CNN affiliate KDVR reported.

The Colorado Department of Public Safety will be joining the investigation and may use "both ground-based teams and aircraft," the agency wrote. State emergency management officials said they won't be discussing more details about their operations at the time.

Mysterious drones have been flying over Colorado and Nebraska in recent weeks and authorities have not been able to figure out much about who may be controlling them.

Colorado deputies have spotted more than a dozen unmanned drones flying in the northeast part of the state.

Witnesses say the drones mostly appear after sunset; sometimes there are dozens at a time and they move in formation. They fly 200-300 feet above ground and some have wingspans as large as 6 feet wide.

Yuma County Sheriff Todd Combs has said the aircraft are operating within federal guidelines but are making residents "very nervous and anxious."

On Monday, a state multi-mission aircraft, known as MMA, flew for 4.8 hours in northeast Colorado as part of the investigation, officials said in a statement.

"The MMA has the capability to detect heat signatures; it did not detect any suspicious heat signatures or drones related to the drone reportings during its flight. During the flight of the MMA, every report of a suspicious drone was investigated and was resolved, and none was substantiated," the statement said.

The MMA typically flies 10,000 feet above ground.

Earlier this week, the Phillips County Sheriff's Office called for assistance from the public in locating a drone command vehicle but days later, the office posted on Facebook that its request was "no longer pertinent or relevant."