WASHINGTON, Feb 8, (AP): The Army was one of 28 government agencies authorized to fly helicopters near Ronald Reagan National Airport before its Black Hawk helicopter collided with an American Airlines passenger jet last week, killing 67. That constant helicopter traffic near such a busy airport is one of many aspects now under review by investigators.

As the wreckage of both aircraft are recovered, authorities are also looking at why the helicopter may have been flying above a 200-foot flight restriction. More details are becoming clear, including that there was no rule on how far inland or over the water the helicopter could be. Here’s what investigators have found, including a new interview with a key Army aviation official: The 12th Aviation Battalion based at Davison Army Air Field at Fort Belvoir, Virginia, has a classified mission to ensure continuity of government by getting certain officials to safety in case of an attack.

It is also tasked with ferrying high-ranking government and military officials to bases throughout the region. The battalion is stretched thin, both for aircraft and personnel. It also flies older Black Hawks. Of the roughly 100 older UH-60L Black Hawk variants left in the Army’s fleet of 2,000 Black Hawks, eight are assigned to the battalion.

After the crash, there are seven. In addition, the unit has six newer UH-60M models known as "gold tops.” The crew was likely wearing night vision goggles at the time of the crash, National Transportation Safety Board chairwoman Jennifer Homendy told reporters on Capitol Hill Thursday. Homendy said investigators listening to the recovered cockpit voice recorder do not hear the crew say they are flying "unaided” - which they would have indicated if they did not have goggles on.

In an interview Thursday with The Associated Press, Brig Gen Matthew Braman, the director of Army aviation and a former UH-60 Black Hawk pilot, said crews train to fly at night with or without the goggles. Whether to wear them is the call of the pilot in command, but if they are worn, all three crew members will have them on.

"Army policy says that as a crew, you will either all be goggled up or you’ll all de-goggle together,” Braman said. Night vision goggles can make depth perception and determining the closing distance to an object more difficult as the lights appear as bright blooms that can be hard to distinguish. Crews train to handle those challenges, Braman said, and are taught to cross-check distance, such as by briefly looking out from under the goggles.