Federal and airport authorities said Friday they are investigating why an air traffic controller became incapacitated and went silent while working a night shift alone in the tower at busy McCarran International Airport in Las Vegas.

"No safety events occurred during this incident," the Federal Aviation Administration said in a statement about what officials said amounted to a 40-minute span during which the female controller slurred words and then apparently lost consciousness shortly before midnight Wednesday.

"An air traffic controller at the Las Vegas tower became incapacitated while on duty," the agency said. It did not identify the controller or the cause of her incapacitation.

Airport director Rosemary Vassiliadis issued a statement saying that initial findings echoed the FAA assessment.

Five inbound aircraft remained airborne during the incident, and aircraft on the ground held positions or communicated between themselves to maintain safety while moving, the FAA said.

The controller involved in the Las Vegas incident is no longer employed by the FAA, the agency told NBC News Sunday.

Air traffic recordings available on the internet show commercial airline pilots having trouble understanding the controller during radio communications about approaches to land, clearances to take off and directions for taxiing. Some begin talking between themselves about something being amiss.

At one point, the controller sounds sleepy and apologizes over the radio, saying she is "choking a little bit." Minutes later, she misstates an aircraft's call numbers. Finally, her microphone opens to the sound of coughing and grunting.

She does not respond to a pilot's inquiry before the sound of a male voice is heard in the room asking if the woman is all right.

Officials said a male controller who had been on break was summoned to return to the tower. Paramedics responded.

The FAA said the woman was at first put on administrative leave, and the agency ordered two controllers to be in the tower during busy hours.

"The FAA is deeply concerned by the incident, is thoroughly investigating what occurred, and is taking immediate steps to modify its overnight shift staffing policies," the agency statement said.

McCarran is among the 10 busiest airports in the U.S. in passenger volume. The unidentified controller worked for a little more than an hour before trouble began and communicated with pilots of 29 aircraft before she was replaced, the FAA said.

National Air Traffic Controllers Association chief Paul Rinaldi issued a statement praising the work of thousands of union members around the country and promising cooperation in the FAA investigation "so that all of the facts are known."