NEW ORLEANS - A pivotal moment in the Gulf oil crisis hit an unexpected snag Tuesday evening when officials announced they needed more time before they could begin choking off the geyser of crude at the bottom of the sea.

BP and federal officials did not say what prompted the decision or when the testing would begin on a new cap it had just installed on the blown-out well. The oil giant had been scheduled to start slowly shutting off valves on the 75-ton cap, aiming to stop the flow of oil for the first time in three months.

It seemed BP was on track to start the test Tuesday afternoon. The cap, lowered over the blown-out well Monday night, is 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 can finally begin to 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 holds or any new leaks erupt.

The operation could last anywhere from six to 48 hours.

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 through pipes to as many as four collection ships.

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.

In Washington, White House spokesman Robert Gibbs said the effort to put the containment cap into operation "represents the best news that we've had in the preceding 85 days."

"We are approaching what we hope is the next phase in the Gulf - understanding that that next phase is likely to take many years," he added.

BP engineers planned to shut off pipes that are already funneling some oil to two ships, to see how the cap handles the pressure of the crude coming up from the ground. Then they planned to close, one by one, three valves that let oil pass through the cap.

Experts said stopping the oil too quickly could blow the cap off or further damage the well.

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.

"What we can't tell is the current condition of the wellbore below the seafloor," Allen said. "That is the purpose of the well integrity test."

If the cap cannot handle the pressure, or leaks are found, BP will have to reopen the valves and let some of the oil out. In that case, BP is ready to collect the crude by piping it to as many as four vessels on the surface.

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.