(CNN) -- "Multiple companies and work teams" contributed to the April 20 explosion aboard the offshore Deepwater Horizon oil rig, unleashing the worst oil spill in U.S. history, BP's internal investigation of the disaster concluded Wednesday.
While BP said its team "incorrectly accepted" results of a negative pressure test aboard the rig before the blast, the internal report assigns much of the blame to rig owner Transocean.
"Over a 40-minute period, the Transocean rig crew failed to recognize and act on the influx of hydrocarbons into the well until the hydrocarbons were in the riser and rapidly flowing to the surface," the report states.
The explosion left 11 men dead and spewed millions of barrels of oil into the Gulf of Mexico over an 87-day period.
BP investigators found signs of "potential weaknesses in the testing regime and maintenance management system" for the blowout preventer that failed to shut down the well at the heart of the underwater oil gusher, the report said.
"The team did not identify any single action or inaction that caused this accident," according to the report. "Rather, a complex and interlinked series of mechanical failure, human judgments, engineering design, operational implementation and team interfaces came together to allow the initiation and escalation of the accident."
Meanwhile, a federal task report Tuesday said scientists have found a decline in oxygen levels in the Gulf following the BP spill, but no "dead zones."
Levels of dissolved oxygen in deep water have dropped about 20 percent below their long-term average, according to data collected from up to 60 miles from the well at the center of the worst oil spill in U.S. history.
But much of that dip appears to be the result of microbes using oxygen to dissolve oil underwater, and the decline is not enough to be fatal to marine life, said Steve Murawski of the National Oceanic and Atmospheric Administration and the head of the Joint Analysis Group studying the spill's impact.
"Even the lowest observations in all of these was substantially above the threshold," Murawski said.
The samples were collected from 419 points at varying distances from the ruptured well at the heart of the disaster and at depths as far down as 4,800 feet, the group reported. The task force is made up of NOAA, the Environmental Protection Agency and the White House Office of Science and Technology Policy.
The undersea gusher erupted in April, releasing an estimated 4.9 million barrels (205 million gallons) of crude before being temporarily capped in July. The volume of oil -- and the amount of chemical dispersants used to break it up -- have created concerns about the long-term health of the Gulf.
The spill also delivered an economic blow to the region, where fisheries and beach resorts are major employers.
Early findings from a mid-August survey led by the University of South Florida indicated oil had settled to the bottom of the Gulf farther east than previously suspected and at levels toxic to marine life.
At about the same time, a team from Georgia Sea Grant and the University of Georgia released a report that estimates that 70 to 79 percent of the oil that leaked from the well "has not been recovered and remains a threat to the ecosystem."
The latest study "does not discuss the broad ecosystem consequences of hydrocarbons released into the environment," NOAA said. But it concludes that the oil is continuing to break up and disperse underneath the surface, making the emergency of a major oxygen-poor dead zone unlikely.
In early August, the federal government estimated that three-quarters of the oil spilled had either evaporated or been dispersed, or had been skimmed or burned off the surface.
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