Federal investigators have concluded their four-day hearing into the cause of a Continental jet crash that killed 28 people, but their findings probably won't be released until sometime around the accident's first anniversary in November. ``We have not eliminated any issue'' in the suspected cause of the Nov. 15 crash of Flight 1713, board of inquiry chairman Joseph T. Nall said Friday after investigators heard the final testimony in the National Transportation Safety Board hearings. Flight 1713, bound for Boise, Idaho, began rocking violently moments after taking off from Stapleton International Airport during a snowstorm. It flipped over on its back and crashed on the runway, breaking into three pieces. Fifty-four of the 82 people on board survived. Some of those survivors were among the 33 witnesses who testified at the hearing. The NTSB investigators were looking into flight characteristics of the DC-9-10 aircraft, especially in snow and ice conditions; air traffic control; cold weather operations by Continental; turbulence; and Continental's pilot training, pilot selection and crew pairing. ``I anticipate our board will determine a probable cause and contributing causes to the crash'' by November, Nall said. The pilot and co-pilot, Capt. Frank Zvonek, 43, and First Officer Lee Bruecher, 26, both were killed in the crash. Bruecher was at the controls. The brief flying time in the DC-9 by both pilots has been one area of study by the investigation. Zvonek had 12,135 total hours flying time, with 33 hours as a DC-9 captain. Bruecher had a total of 3,186 hours, with 12 hours as a DC-9 first officer. At Friday's hearings, Richard Hillman, Continental vice president for flight operations, said that since the crash, the carrier had adopted a Federal Aviation Administration bulletin recommendation that the co-pilot not be allowed to make takeoffs or landings until he or she had 100 hours in that type of aircraft. The hearings also delved into Continental's use of a flight simulator in pilot training that is based on the DC-9-30 _ a different model of DC-9 from the ``Dash 10'' flown by Zvonek and Bruecher. The main difference between the two DC-9 models is the ``Dash 30'' has wing slats to improve the wing's lift. The Dash 10 is an older model with a wing design considered more susceptiable to loss of lift from ice buildup. James E. Madsen, Continental's assistant chief DC-9 pilot at Denver, minimized handling difference between the Dash 10 and Dash 30s in his testimony. ``A DC-9 is a DC-9. There are some differences they (pilots) should be aware of,'' Madsen said. Early theories of what caused Flight 1713 to roll violently and crash seconds after liftoff included ice buildup on the wings, turbulence from a jumbo jet that had just landed on a parallel runway 1,600 feet away, and pilot error. Continental's de-icing procedures at Stapleton also came under scrutiny. Flight 1713 waited between 24 and 27 minutes for takeoff after it had been de-iced. Survivors offered conflicting testimony on the amount of snow and ice buildup on the wings. ``I wondered at one point, `Is that ice?' But I decided it was just wet,'' said Dr. Fred Helpenstall of Nampa, Idaho, who suffered broken ribs in the crash. Airman Greg Wadsworth of Mountain Home Air Force Base in Idaho said he had a clear view of the wing before takeoff. ``It was like the snow was starting to pile up (on the wings). I was very concerned but I said it under my breath,'' he testified. Other testimony attributed Flight 1713's delay between de-icing and takeoff on miscommunciation in the control tower and between the tower and aircraft. NTSB investigators refused to speculate on any of the suspected causes. ``Our minds are still open. We will make no final conclusion until the time we make our final report,'' Nall said.