NEW : NTSB investigators looking for video of bus driver 's casino activities before the accident . Comment: Driver of tractor-trailer disputes bus driver account of crash involvement , police source says . Witnesses say bus was traveling very fast before accident that killed 15 .
NEW : NTSB investigators looking for video of bus driver 's casino activities before the accident . Witnesses tell police bus rolled over rumble strips . Driver of tractor-trailer disputes bus driver account of crash involvement , police source says . Witnesses say bus was traveling very fast before accident that killed 15 .