BP had three indications of trouble aboard the doomed drill rig Deepwater Horizon in the hour before the April 20 explosion that sank the offshore platform, congressional investigators reported Tuesday.

The memo summarizes preliminary findings of BP's own investigation into the disaster, which left 11 workers dead and uncapped an undersea gusher that has spewed crude oil into the Gulf of Mexico for a month.

The well unexpectedly spouted fluid three times in the 51 minutes before the explosion and pressure on the drill pipe "unexpectedly increased" before the blast, the memo states. In addition, BP's investigation "raised concerns about the maintenance history, modification, inspection, and testing" of blowout preventer, a critical piece of equipment that has failed to shut down the well.