Federal investigators are looking at weather conditions, wreckage and other evidence as they work to determine what caused a Learjet 60 to crash in Palm Beach County, killing all nine people on board.The National Transportation Safety Board said Monday that weather conditions are a major focus of its investigation. Investigators also said there is some evidence the aircraft began breaking apart before impact, but they are still working to determine what happened.WATCH NOW: West Palm Beach plane crash puts aviation safety, recent Tampa Bay crashes in focus West Palm Beach plane crash puts aviation safety, recent Tampa Bay crashes in focusWhat investigators know so farAccording to NTSB officials, the Learjet 60, operated by Aurora Aviation doing business as UpperJets, left Austin at 10:37 a.m. Sunday bound for the Bahamas. Seven passengers and two flight crew members were on board.At 12:47 p.m., the crew reported encountering white and clear ice that they described as manageable, along with moderate turbulence, according to the NTSB. The aircraft was flying at 35,000 feet, where investigators said the outside temperature was minus 53 degrees Fahrenheit.The crew also reported picking up precipitation and requested a change in altitude. About 30 seconds later, investigators said the crew declared "Mayday" and reported an overspeed condition before the remainder of the transmission became unintelligible.NTSB Board Member Tom Chapman stressed that investigators have not yet determined exactly what role the weather played.The weather conditions are a major focus of the investigation, Chapman said. Investigators also have evidence they're examining that could provide more information about what happened before the plane hit the ground.We do have some evidence that there was some breakup of the aircraft before impact, Chapman said. We're still seeking to determine that.The NTSB said it could not yet confirm whether the aircraft flew through a thunderstorm.Aviation experts explain what investigators will examineAviation attorney John Gagliano, who is board certified in aviation law by the Florida Bar and is also a former U.S. Navy pilot and mechanical engineer, said determining what happened requires examining numerous potential factors.There are so many things that need to be looked into, from the aircrew to the maintenance to the weather, the manufacturer, prior problems with the plane, what type of autopilot was installed, how everything was working recently, Gagliano said. Gagliano cautioned against drawing a conclusion about the cause solely from the weather conditions reported by the crew. We don't know how much weather was a factor yet, Gagliano said. We know that there was precipitation and turbulence. That could be the cause of the crash, but it could also have nothing to do with the cause of the crash.What are pilots trained to do when they encounter icing?Retired U.S. Navy aviator Richard Kolko said pilots are trained to respond when they believe their aircraft is entering conditions where icing could occur.If you think you're entering into an icing area or icing could occur on your aircraft, you have to make changes, Kolko said.Kolko explained that aircraft can have both anti-ice and de-ice systems. He said anti-ice systems are designed to help prevent ice from accumulating, while de-ice systems can help remove ice after it begins forming. He also said pilots have to be familiar with those systems and monitor conditions before and during flight.The NTSB has not determined whether icing caused or contributed to Sunday's crash.Putting the crash into perspective in Tampa BayThe West Palm Beach crash also raises questions closer to home.Tampa Bay 28 has covered at least three small-aircraft crashes in the Tampa Bay area this year, two of which were deadly. Zarcone asked Kolko whether smaller aircraft are more susceptible to crashes.Kolko emphasized the broader safety record of commercial air travel.Flying in a commercial-size aircraft is about the safest way to travel you can in the United States, Kolko said.While Kolko said accidents involving smaller aircraft do happen, the interviews conducted for this story did not provide statistical data establishing whether those accidents are becoming more frequent. The recent Tampa Bay crashes therefore should not, on their own, be characterized as evidence of a broader trend.Investigation continuesNTSB investigators said recovering wreckage is one of the major challenges at the crash site. According to investigators, the debris field stretches approximately half a mile by four-tenths of a mile and includes areas around ballfields, businesses and homes.Investigators said water in the impact crater and continuing weather conditions are complicating recovery efforts.The NTSB is also asking witnesses to come forward and urging members of the community who find possible debris not to move it.Chapman said the agency's goal goes beyond determining what happened.Our mission is to understand not just what happened, but why it happened and to recommend changes to have it not happen again. ...read more read less