Retired Coast Guard Adm. Thad Allen, the government's point man for the spill response, said crews must move forward drilling the relief well even though officials were still evaluating the best way to finish it.
"The relief well will be finished," he said. "We will kill the well."
BP had thought the mud and cement pumped in from above the leak may have essentially killed the well. But the relief well will allow engineers to pump in mud and cement from below, which is intended to permanently seal the well.
Work on the wells was stopped this week because of bad weather.
The decision to resume work on the relief wells means a key milestone in the crisis that wreaked havoc on the Gulf Coast's economy and ecosystem remains days off. However, Allen has repeatedly insisted on an "overabundance of caution" when it comes to permanently plugging the well.
Officials had been testing the pressure beneath the cement plug currently in place. Steady pressure would indicate the presence of cement in the space between the inner piping and the outer casing, likely indicating a permanent seal.
But because pressure rose during the testing, the scientists concluded that space still needs to be plugged in.
It would have been difficult to say the "bottom kill" was unnecessary after promising it for weeks as the ultimate solution, said Eric Smith, associate director of the Tulane Energy Institute.
"That's been the mantra all along, that they wanted to do the bottom kill," he said.
Proceeding with the relief well makes sense, said Bob Bea, a petroleum engineering professor at the University of California, Berkeley, who is conducting his own investigation into the disaster.
Too little is known about how much cement might be inside the space between the inner piping and outer casing to be confident the well is permanently plugged, he said.
"Everything we know at this time says we need to continue the work with the relief wells," he said. "We don't know the details of how they plugged the well from the top. We don't know the volume of material they put in the well bore, and without that we can't tell how close to the bottom of the well they got."
Drilling of the first relief well began in early May. Since then, the drill has been guided some three miles from the surface and two miles beneath the sea floor to within 30 to 50 feet of the target. The drill is about as wide as a grapefruit, its target less than half the size of a dartboard.
It's unclear when the drilling could be finished. Officials had projected as early as Friday before the nasty weather forced the operation to a halt. Drilling that final stretch is a time-consuming and careful process as engineers work to make sure they don't miss.
Crews dig about 20 to 30 feet at a time, then run electric current through the relief well. The current creates a magnetic field in the pipe of the blown-out well, allowing engineers to calculate exactly where and how far they need to drill.
The flow of oil into the Gulf has been halted since July 15, when a temporary cap over the well was able to contain the spill. But officials have stressed for weeks that only a bottom kill will ensure the well is no longer a danger.
Before July 15, the oil leaked almost unimpeded for nearly three months and spewed some 206 million gallons of oil, according to the government's latest estimate. The crisis began on April 20, after an explosion on the BP PLC-leased Deepwater Horizon drilling rig that killed 11 workers. Officials don't yet know the cause of the explosion, or why machinery designed to prevent the unchecked flow of oil failed to work.
BP has already spent $6.1 billion responding to the spill.