Austin, Texas (CNN) -- The man who flew an airplane into a building housing an Internal Revenue Service office may have replaced some of its seats with a drum of fuel to cause maximum damage, a law enforcement official familiar with the investigation said Friday. The official, who would not speak on the record because it is an ongoing case, said investigators have determined that the Piper Cherokee PA-28 had several seats removed and that a fuel drum was missing from the airport from which Andrew Joseph "Joe" Stack III took off Thursday morning. "I think there is a good chance he might have put it on his plane," said the official, who cautioned that investigators were still working that lead and sifting through the crash site.
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NEW: Seats removed from plane, fuel drum missing from airport
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FBI takes lead role in investigation into Austin, Texas, crash
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Pilot's wife expresses her "sincere sympathy to the victims and their families"
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Joseph Stack believed to have flown plane into IRS building in suicidal attack
Friends and former colleagues said Friday they had no inkling of the rage apparently building inside @placeholder.