Federal authorities are investigating the near-collision of two passenger jets over Iowa this week that involved air traffic controllers serving O'Hare International Airport, the Federal Aviation Administration said Thursday.One of the jets was headed for Denver International Airport. The jets were less than two miles apart and headed toward each other at 500 mph when on-board collision alarms went off, leading both airplanes to take evasive action, the Chicago Tribune and WBBM-TV reported on their Web sites, citing the FAA and air traffic controllers. A call placed to the FAA by The Associated Press late Thursday was not immediately returned. A controller also helped prevent the planes -- flying at about 30,000 feet -- from crashing into each other, according to the controllers union. The Tuesday incident involved a United Express commuter jet headed to O'Hare from Lincoln, Neb. and a Northwest Airlines Boeing 757-- about 900 feet higher -- en route to Denver from Detroit, the FAA said. The FAA is investigating the incident as a violation of a requirement for a 1,000-foot minimum vertical separation between planes. The federal agency attributed the near-collision to a mistake by a controller at the agency's Chicago Center radar facility in Aurora, Ill.; the center directs high-altitude traffic over sections of the Midwest. The National Air Traffic Controllers Association, however, says the controller responsible for the planes and an assistant became distracted by a new technology being tested at the time. "The planes were closing in on each other and the controller took action to keep the planes separated," Toby Hauck, union vice president at the Aurora center, said. The FAA, however, said the controller wasn't working with the new technology. "The controller lost track of where the planes were and they lost separation," FAA spokesman Tony Molinaro said. The investigation of the incident is continuing, he said.