A permanent cement plug sealed BP's well nearly 2.5 miles (four kilometers) below the sea floor in the Gulf of Mexico, five agonizing months after an explosion sank a drilling rig and led to the worst offshore oil spill in U.S. history.

Retired Coast Guard Adm. Thad Allen, the federal government's point man on the disaster, said Sunday that BP's well "is effectively dead." Allen said a pressure test to ensure the cement plug would hold was completed at 5:54 a.m. CDT (1054 GMT).

He said the federal Bureau of Ocean Energy Management Regulation and Enforcement has confirmed that the cementing operation was successful.

"Additional regulatory steps will be undertaken but we can now state definitively that the Macondo Well poses no continuing threat to the Gulf of Mexico," Allen said.

The gusher was contained in mid-July after a temporary cap was successfully fitted atop the well. Mud and cement were later pushed down through the top of the well, allowing the cap to be removed.

But the well could not be declared dead until a relief well was drilled so that the ruptured well could be sealed from the bottom, ensuring it never causes a problem again. The relief well intersected the blown-out well Thursday, and crews started pumping in the cement on Friday.

The April 20 blast killed 11 workers, and 206 million gallons (780 million liters) of oil spewed into the Gulf.

The disaster caused an environmental and economic nightmare for people who live, work and play along hundreds of miles (kilometers) of Gulf shoreline from Florida to Texas. It also spurred civil and criminal investigations, cost gaffe-prone BP chief Tony Hayward his job, and brought increased governmental scrutiny of the oil and gas industry, including a costly moratorium on deepwater offshore drilling that is still in place.

Gulf residents will be feeling the pain for years to come. There is still plenty of oil in the water, and some continues to wash up on shore. Many people are still struggling to make ends meet with some waters still closed to fishing. Shrimpers who are allowed to fish are finding it difficult to sell their catch because of the perception _ largely from people outside the region _ that the seafood is not safe to eat. Tourism along the Gulf has taken a hit.

The disaster also has taken a toll on the once mighty oil giant BP PLC. The British company's stock price took a nosedive after the explosion, though it has recovered somewhat. Its image as a steward of the environment was stained and its stated commitment to safety was challenged. Owners of BP-branded gas stations in the U.S. were hit with lost sales, as customers protested at the pump.

And on the financial side: BP has already shelled out more than $8 billion in cleanup costs and promised to set aside another $20 billion for a victims compensation fund. The company could face tens of billions of dollars more in government fines and legal costs from hundreds of pending lawsuits.

BP took some of the blame for the Gulf oil disaster in an internal report issued earlier this month, acknowledging among other things that its workers misinterpreted a key pressure test of the well. But in a possible preview of its legal strategy, it also pointed the finger at its partners on the doomed rig.

BP was a majority owner of the well that blew out, and it was leasing the rig that exploded from owner Transocean Ltd.