DENVER -- Federal authorities investigating the crash of a corporate jet near the Pueblo airport that killed all eight people aboard said the plane's approach appeared normal until less than a minute before the crash. That's when the Cessna Citation C-560 descended 1,364 feet in 30 seconds.

The pilot did not radio the control tower to say what happened, according to Frank Hilldrup, the National Transportation Safety Board investigator in charge of the crash investigation.

During a typical landing over flat terrain, an aircraft descends at a rate of 500 feet-per-minute before reaching the runway. Based on altitude information released by investigators, the rate of descent for the computer jet was approximately 2,600 feet-per-minute during the final 30 seconds of the flight.

Hilldrup and NTSB chairwoman Ellen Engleman Conners said investigators had interviewed air traffic controllers at Pueblo Memorial Airport.

Hilldrup said the last radar reading of the plane indicated it was about 300 feet above the ground, 15 seconds before impact.

Before the plane's fast drop, its "descent profile looked OK," Hilldrup said.

Investigators sent the cockpit voice recorder to Washington for analysis. They also recovered a device that warns if a plane is too close to the ground, National Transportation Safety Board spokeswoman Lauren Peduzzi said. Investigators also removed the voice recorder from the other Cessna Citation, which was also owned by Circuit City, that landed safely just minutes after the first one crashed.

Earlier Thursday, Conners had said radar tracking of the plane had revealed some "interesting characteristics" but did not elaborate.

The jet was stopping to refuel in Pueblo when it crashed. Circuit City said four employees, two outside business partners and two pilots were killed.

Neither Circuit City nor the Pueblo County coroner has released their names but families confirmed that the victims included pilot Bruce Walton, 53, of Richmond, Va.; co-pilot Jeffrey Wightman, 42, of Tappahannock, Va.; Kyle Jeffrey Harmon, 26, an assistant buyer with Circuit City from Virginia; Aaron Iskowitz, also of Virginia, and Vincent Choe, 32, of New Jersey, The Denver Post reported.

FINAL SECONDS 9:12:30 a.m. jet at 6,100 feet above sea level 9:12:34 a.m. 6,000 feet 9:12:39 a.m. 5,600 feet 9:12:44 a.m. 4,900 feet (airport elevation: 4,736 feet) 9:13 first 911 call comes in

Conners said the investigation will include the weather and potential mechanical problems.

Hilldrup said aviation officials had issued a warning about icing conditions in the Pueblo area before the crash, and the pilot of the other Circuit City jet that landed shortly after the accident reported light to moderate icing.

That pilot activated an in-flight deicing system. Ice can weigh down a plane and hurt its aerodynamics.

Hilldrup said it was too early to say whether the plane's jet engines were operating normally when the aircraft crashed.

Investigators were interviewing witnesses and getting records on the aircraft and its crew.

Two witnesses told sheriff's investigators they heard loud popping noises from plane shortly before the crash at about 9 a.m. Conners said investigators were looking into those accounts.

The flight originated in Richmond, Va., home of Circuit City, and was heading to Orange County, Calif. authorities said. The planes made a refueling stop early Wednesday in Columbia, Mo.

It was the fourth fatal crash of a small plane in Colorado since late November.

A Cessna 560 crashed in 1995 in Eagle River, Wis., killing two pilots, the only people on board. NTSB spokeswoman Lauren Peduzzi said it would be "very premature" to draw parallels between the two incidents.