NEW : The cause of the accident is not clear ; NTSB will investigate . Officials believe no toxic inhalants were aboard . CNN iReporter Charlotte Curtis sent video of thick smoke with sirens wailing in the background . UPDATED : Witness : 'Deafening ' explosion followed derailment .
NEW : The cause of the accident is not clear ; NTSB will investigate . Officials believe no toxic inhalants were aboard . UPDATED : Witness : 'Deafening ' explosion followed derailment .