A small jet owned by electronics retailer Circuit City (search) crashed in freezing drizzle Wednesday as it approached a southern Colorado airport, killing all eight people aboard, including four company employees.

Two witnesses told investigators they heard loud popping noises from the twin-jet Cessna Citation C-560 (search) shortly before the crash at about 9 a.m., Pueblo County Sheriff Dan Corsentino said. The cause of the crash was unknown.

"I don't have any idea why it went down. It is just an unfortunate thing," sheriff's spokesman Steve Bryant said. A National Transportation Safety Board official was at the scene and a team of investigators was expected to arrive late Wednesday.

FAA spokesman Mike Fergus said the pilot was relying on the plane's instruments to make the Pueblo airport approach because of poor weather. The National Weather Service (search) reported low clouds, fog and freezing drizzle with visibility of about six miles at the airport at the time. The temperature was 27 degrees.

In 1998, the FAA required operators of several Cessna models, including the one that crashed Wednesday, to add a warning to their flight manuals that flying in freezing drizzle and other conditions "may result in ice build-up on protected surfaces exceeding the capability of the ice protection system." uth of Denver, on sagebrush-dotted grassland between Pueblo and the Army's sprawling Pueblo Chemical Depot. A small fire was quickly extinguished.

It was the fourth fatal crash of a small plane in Colorado in the past three months. A Nov. 28 crash in Montrose in southwestern Colorado killed three people, including the 14-year-old son of NBC Sports executive Dick Ebersol. Authorities are investigating whether ice on the wings was to blame.

In December, two planes crashed a week apart at Centennial Airport in suburban Denver. Two people were killed in each crash.