DENVER -- A driver violated every procedure and training protocol when he drove a snowplow onto a runway and in front of a United Airlines plane landing at Denver International Airport, airport officials said this week in the wake of a federal report.

A National Transportation Safety Board concluded the plow driver crossed the runway "without clearance from air traffic control or airport operations."

Pilots of the Boeing 737, which was carrying 96 passengers and five crew members on a flight from Billings, Mont., used significant reverse thrust and brakes to stop the aircraft and no one was injured.

"The driver said he saw the landing airplane as he was crossing the runway and increased acceleration," the NTSB factual report said of the Feb. 2 incident.

Another vehicle that was in radio contact with the air control tower was escorting the plow, but it had cleared the runway by the time the plane landed, the NTSB's report said.

It was unclear whether the 42-year-old plow operator was in radio contact with either the escort or the tower.

Pilots of the United plane said they saw the plow "holding short" of a taxiway that runs parallel to the runway. "They landed and during the rollout, they observed the snowplow cross the runway in front of them," the federal safety report said.

"Appropriate personnel action was taken, and he no longer works for the city," DIA spokeswoman Sally Covington said of the plow driver. The driver's name was not released and he could not be contacted for comment.

DIA officials said they have made several changes, including making its training program more rigorous, hired more trainers, and added warning signs at key intersections.

DIA also is trying to reduce the number of employees cleared to drive on the runways.

The incident was the second in less than a month in February and January. On Jan. 5, a Frontier Airlines jet attempting to land in Denver came within 50 feet of a smaller charter plane that had inadvertently entered the runway.

The Frontier crew spotted the Key Lime Air plane in time, aborted the landing and continued flying until it could land the jetliner a short time later, authorities said. No injuries were reported.

The NTSB is also investigating that incident.