The ruptured well which pumped almost five million barrels of oil into the Gulf of Mexico has been secured and no longer constitutes a threat, it has been confirmed.
A new valve known as a blowout preventer was placed over the well on Friday after crews replaced a damaged device.
This will now be examined by investigators looking into the causes of the disaster.
Incident commander Admiral Thad Allen said: "Under the direction of the federal science team and US government engineers, BP used the Development Driller II to successfully install a fully functioning and tested blowout preventer (BOP) on the cemented Macondo 252 well."
He said there was no apparent release of hydrocarbons during the operation.
"This is an important milestone as we move toward completing the relief well and permanently killing the Macondo 252 well. I will continue to provide updates as necessary," he added.
The ruptured well has been shut since July 15. The Deepwater Horizon rig exploded on April 20, killing 11 workers and causing the worst environmental disaster to hit the region.
Since the accident BP has come under prolonged attack in the US for perceived safety failings and its attitude towards the investigation.
Besides causing catastrophic pollution, the oil spill also led to the departure of BP chief executive Tony Hayward.
The oil giant has estimated the cost of tackling the disaster to eight billion dollars (£5.2 billion) so far.