They especially wanted to know if pilots had been given accurate information about the threat of ice as they passed through snow and clouds. An icing advisory warned of a light to moderate threat.
The victims, which included two Oklahoma State basketball players and six staffers, were killed Saturday when the twin-engine plane went down in a fiery crash about 40 miles east of Denver.
The plane had taken off from Jefferson County Airport in light snow and with a visibility of one mile.
The crew of the Beechcraft King Air 200 Catpass were told ice could form on the wings, but investigators said conditions were not harsh enough for authorities to ground the plane. It was not known if the plane was de-iced before it took off.
"It's better not to fly any airplane in icing conditions with the exception of a plane with a heated wing," said Jim Burnett, former chairman of the National Transportation Safety board.
National Transportation Safety Board spokesman Paul Schlamm said investigators did not think the plane had a flight data or voice recorder. Such devices, which record the pilot and co-pilot's conversations and the airplane's control settings, were not required on this aircraft under FAA regulations.
No distress call was made before the crash, which happened after the plane leveled off at 23,000 feet.
Burnett told The Associated Press the debris, which is in a narrow line about a mile long tilting towards the southwest, was not consistent with a nose dive.
The line of debris could mean the airplane broke up in the air, which could have been caused by the airplane flying in an unusual position, such as upside down, said Burnett, who served as chairman from 1982 to 1988.
According to investigators, the main fuselage is at the beginning of the main line of debris, with between 100 and 200 yards of debris to the northwest.
John Hammerschmidt, head of the NTSB investigation team, said fractured pieces of metal from the airplane were being examined to determine if they caused the crash or if they were broken during the crash. Other pieces of the wreckage were also being collected.
Investigators are also looking at weather and radar data, flight manuals, maintenance records and interviewing pilots who have flown the airplane.
The plane was one of three planes carrying the school's basketball team and associates back to Stillwater, Okla., after they lost to Colorado at Boulder.
In Boulder, the black and orange Oklahoma State flag that normally flies with the other Big 12 ones on football game days at Folsom Field was hoisted and flying at half mast.
The plane was registered to North Bay Charter of Reno, Nev. The company declined to comment on Monday.
Among the victims were Oklahoma State players Nate Fleming and Daniel Lawson, sports information employee Will Hancock, director of basketball operations Pat Noyes and their trainer Brian Luinstra.
Witnesses said the plane climbed and banked hard to the right before it crashed. They told investigators the propeller plane's engines revved and eased several times before it crashed.
Also killed in the crash was student manager Jared Weiberg, the nephew of Big 12 commissioner Kevin Weiberg, broadcast engineer Kendall Durfey, broadcaster Bill Teegins, pilot Denver Mills and co-pilot Bjorn Fahlstrom.
Oklahoma State University Vice President David Bosserman was in Colorado to help transport the bodies back home. There were no plans to bring relatives to the crash site.
The university was planning a memorial service 3 p.m. Wednesday at the school.
------ On the Net: NTSB: http://www.ntsb.gov
------ FAA: http://www.faa.gov