NEW ORLEANS — BP’s blown-out oil well in the Gulf of Mexico has been killed, five months after an explosion sank a drilling rig and led to the worst offshore oil spill in US history.

Retired Coast Guard Admiral Thad Allen, the federal government’s point man on the disaster, said yesterday that BP’s Deepwater Horizon well “is effectively dead.’’

Crews pumped in cement Friday to plug the well nearly 2.5 miles below the sea floor. The mixture had hardened by Saturday, and a pressure test completed early yesterday confirmed that the plug would hold.

The official end of the well occurred 11 years after Texaco first sank an exploratory well near that same spot 50 miles out in the Gulf of Mexico, then moved on after finding it unprofitable. BP purchased the rights to explore for oil there in 2008.

On April 20, an explosion on the rig — which had drilled the well and was in the process of capping it — killed 11 men instantly and started a disaster that has jeopardized the livelihoods of legions of fishermen, hotel and restaurant workers, drilling employees, and others.

In the three months before a temporary cap stemmed the flow from the blown-out well in July, an estimated 206 million gallons of oil spewed into gulf waters.

For those most directly affected by the spill — the ones who still await BP checks for lost wages and revenues, who live on beaches where oil mats are just now coming ashore — the feeling of helplessness remains.

“If you had to live with all the uncertainty, for all those months,’’ said Mike Helmer, a fishing guide in Lafitte, La. “I can promise you it’s not easy. And it’s not over.’’

BP’s internal investigation, released earlier this month, pointed blame in many directions. It accused subcontractor Halliburton of improperly cementing the well. It blamed rig owner Transocean Ltd. for problems with the blowout preventer on the sea floor a mile down.

It even pointed at itself, acknowledging that if the results of a critical pressure test had been correctly interpreted, workers would have known something was horribly wrong in time to do something about it.

While the company’s report went a long way toward previewing its legal strategy and explaining how explosive gas made a 3-mile-plus journey from the bottom of the well to the drilling rig, it left questions unanswered.

Those questions will be addressed by government investigators, other companies’ investigations, congressional committees, and by examinations of key pieces of evidence.

Some of those probes are looking specifically at factors BP downplayed — including the company’s well design.

The conclusions will help determine who is liable and what share of the blame — and of the bill — the companies with ties to the rig and its equipment will be responsible for. Based on an upper estimate of the oil spilled, BP and others could be fined up to $5.4 billion for violating water pollution laws, or up to $21 billion if gross negligence is found.

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