A CareFlite medical helicopter on the way to pick up a patient came "very close" to striking a drone aircraft Monday near Denton, the company's top executive said.

The helicopter had just taken off at about 5:30 p.m. when the incident occurred about four miles south of Denton Regional Airport, said CareFlite Chief Executive Officer Jim Swartz.

"It went right past the aircraft," Swartz said. "I would say it was very close. We were lucky nothing terrible happened."

The pilot, a nurse and a paramedic were in the helicopter at about 500-600 feet above the ground, Swartz said.

It happened so fast, the pilot did not have time to take evasive action, he said. The crew was startled but continued its assigned flight.

"I'm not sure the people who fly these drones know the risk they're putting airplanes and helicopters in," Swartz said. "They're putting them at terrible risk."

CareFlite was established in 1979 and operates six helicopters in North Texas.

Swartz said similar incidents with drones happen every few months but that this was an especially close call.

The pilot reported the scare to the Federal Aviation Administration, he said.

An FAA spokesman said the agency is investigating.

Under federal rules, drone operators are not supposed to fly above 400 feet under any circumstances and must notify the control tower or airport operator if they are within five miles of airports like the one in Denton, the agency said.

"It's a disaster waiting to happen," Swartz said.