The plan to start choking off oil gushing into the Gulf of Mexico was suddenly halted as government officials and BP said further analysis must be done Wednesday before critical tests could proceed.
No explanation was given for the decision, and no date was set for when testing would begin on the new, tighter-fitting cap BP installed on the blown-out well Monday.
In the meantime, oil continued spewing into the Gulf.
The oil giant had been scheduled to start slowly shutting off valves Tuesday on the cap, aiming to stop the flow of oil for the first time in three months. BP was initially ahead of schedule on its latest effort to plug the leak. The cap was designed to be a temporary fix until the well is plugged underground.
A series of methodical, preliminary steps were completed before progress stalled. Engineers spent hours on a seismic survey, creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. It also provides a baseline to compare with later surveys during and after the test to see if the pressure on the well is causing underground problems.
An unstable area around the wellbore could create bigger problems if the leak continued elsewhere in the well after the cap valves were shut, experts said.
"It's an incredibly big concern," said Don Van Nieuwenhuise, director of Professional Geoscience Programs at the University of Houston. "They need to get a scan of where things are, that way when they do pressure testing, they know to look out for ruptures or changes."
It was unclear whether there was something in the results of the mapping that prompted officials to delay. Earlier, BP Vice President Kent Wells said he hadn't heard what the results were, but he felt "comfortable that they were good."
National Incident Commander Thad Allen met with the federal energy secretary and the head of the U.S. Geological Survey as well as BP officials and other scientists after the mapping was done.
"As a result of these discussions, we decided that the process may benefit from additional analysis," Allen said in a statement. He didn't specify what type of analysis would be done, but said work would continue until Wednesday.
Assuming BP gets the green light to do the cap testing after the extra analysis is finished, engineers need to shut off lines already funneling some oil to ships to see how the cap handles the pressure of the crude coming up from the ground.
Finally, they would shut the openings in the 75-ton metal stack of pipes and valves gradually, one at a time, while watching pressure gauges to see if the cap would hold or if any new leaks erupted. The operation could last anywhere from six to 48 hours, once it gets started.
Scientists will be looking for high pressure readings of 8,000 to 9,000 pounds per square inch. Anything lower than 6,000 might indicate previously unidentified leaks in the well.
The oil giant was optimistic about the latest effort after other attempts failed, and White House officials earlier expressed optimism Tuesday.
But BP has said all along they were working carefully so as to not jeopardize the effort to stop the biggest offshore oil spill in U.S. history and one of the nation's worst environmental disasters.
If the cap works, it will enable BP to stop the oil from gushing into the sea, either by holding all the oil inside the well machinery like a stopper or, if the pressure is too great, channeling some though pipes to as many as four collection ships.
Earlier, Allen stressed there were no guarantees on the latest measure and urged patience from Gulf residents.
Along the Gulf Coast, where the spill has heavily damaged the region's vital tourism and fishing industries, people anxiously awaited the outcome of the painstakingly slow work.
"I don't know what's taking them so long. I just hope they take care of it," said Lanette Eder, a vacationing school nutritionist from Hoschton, Ga., who was walking on the white sand at Pensacola Beach, Fla.
"I can't say that I'm optimistic — It's been, what, 84 days now? — but I'm hopeful," said Nancy LaNasa, 56, who runs a yoga center in Pensacola.
The cap is just a stopgap measure. To end the leak for good, the well needs to be plugged at the source. BP is drilling two relief wells through the seafloor to reach the broken well, possibly by late July, and jam it permanently with heavy drilling mud and cement. After that, the Gulf Coast faces a long cleanup.
The leak began after the Deepwater Horizon offshore drilling platform exploded on April 20, killing 11 workers. As of Tuesday, the 84th day of the disaster, between 90.4 and 178.6 million gallons of oil had spewed into the Gulf.
BP underwater video: http://bit.ly/bwCXmR
Weber reported from Houston. Associated Press writers Matt Brown and Tom Breen in New Orleans and Matt Sedensky in Pensacola Beach, Fla., contributed to this report.
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