BP on Thursday removed a massive cap which had stemmed the flow of oil from its ruptured well deep in the Gulf of Mexico in a key step toward killing the well once and for all, officials said.
"Under the direction of the federal science team and US government engineers, BP has completed the capping stack removal procedure," retired Coast Guard Admiral Thad Allen said in a statement.
Allen, the pointman for the US response, called it "an important step in the process to remove and preserve the damaged BOP (blowout preventer)," the large piece of equipment that failed to stop the largest maritime oil spill in history.
Efforts to remove the blowout preventer were set to begin Thursday evening. It will be raised to the surface to be examined and held as evidence in an official investigation.
An estimated 4.9 million barrels of oil gushed into the Gulf of Mexico after an explosion ripped through the Deepwater Horizon rig on April 20, killing 11 workers and sinking the huge offshore platform two days later.
The flow of oil into the sea was not fully stemmed until July 15.
The ruptured Macondo well was plugged with heavy drilling fluid and then sealed it with cement last month but the "bottom kill" operation to permanently seal the ruptured well was delayed until the blowout preventer is replaced.
The bottom kill involves intercepting the crippled well with a relief well, which then pumps heavy drilling oil and cement into the oil well to permanently plug it.
Explore further: US proposes stricter ozone limits