(CNN) -- U.S. officials formally declared an end to the worst oil spill in U.S. history Sunday, a milestone that followed nearly five months of dashed hopes and blistering criticism of nearly everyone involved.

After a weekend of pouring cement into the base of the ruptured well in the Gulf of Mexico, pressure tests conducted early Sunday confirmed the seal was holding, former Coast Guard Adm. Thad Allen said announced. The Interior Department agency that regulates offshore drilling pronounced the well dead at 5:54 a.m. (6:54 a.m. ET), Allen said in a written statement.

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