Engineers struggled to make sense of puzzling pressure readings from the bottom of the sea Friday, trying to determine whether BP's capped oil well was holding tight or in danger of springing a new leak.
No immediate leaks were spotted, which was encouraging.
Allen said BP's test of the cap, which started 24 hours previously by shutting three valves and stopping the flow of oil into the water, would continue for at least 6 hours. It was scheduled to last up to 48 hours.
Allen said two possible reasons were being debated by scientists: The reservoir that is the source of the oil could be running low three months into the spill. Or there could be an undiscovered leak somewhere down in the well. Allen ordered further study but remained confident.
Testing is expected to go on into the night, at which point BP may decide whether to reopen the cap and allow some oil to spill into the sea again.
Two underwater robots scoured the sea floor looking for signs of new leaks. Wells said that one of their main concerns was possible leaking in the pipe under the sea floor.
Wells spoke 17 hours after valves were shut to trap oil inside the cap.
The cap is designed to prevent oil from spilling into the Gulf, either by keeping it bottled up in the well, or by capturing it and piping it to ships on the surface. It is not yet clear which way the cap will be used if it passes the pressure test.
Meantime, work will resume on a relief well, BP's more permanent solution meant to plug the leak for good underground to end one of the nation's worst environmental catastrophes.
Engineers had stopped drilling one of the wells Thursday in case that bore hole deep underground could be affected by the oil cap effort.
The oil giant finally stopped oil from spewing into the sea Thursday for the first time since an April 20 explosion on the BP-leased Deepwater Horizon oil rig killed 11 workers and unleashed the spill 5,000 feet beneath the water's surface.
- Benton F. Baugh, president of Radoil Inc. in Houston and a National Academy of Engineering member who specializes in underwater oil operations, warned that the pressure readings could mean that an underground blowout could occur. He said the oil coming up the well may be leaking out underground and entering a geological pocket that might not be able to hold it.
- But Roger N. Anderson, a professor of marine geology and geophysics at Columbia University, said the oil pressure might be rising slowly not because of a leak, but because of some kind of blockage in the well.
- President Barack Obama said Friday the progress was good news, but cautioned an anxious public not to "get too far ahead of ourselves." Obama said the cap was still being tested and there was still an "enormous clean up job" and ensuring quick compensation for Gulf residents and business in the offing.
- The Gulf Coast has been shaken economically, environmentally and psychologically by the hardships of the past three months. That feeling of being swatted around - by BP, by the government, even by fate - was evident in the wide spectrum of reactions to news of the capping.
- The fishing industry in particular has been buffeted by fallout from the spill. Surveys of oyster grounds in Louisiana showed extensive deaths of the shellfish. Large sections of the Gulf Coast - which accounts for 60 to 70 percent of the oysters eaten in the United States - have been closed to harvesting.
- The saga has also devastated BP, costing it billions in everything from cleanup to repair efforts to plunging stock prices. BP shares, which have lost nearly half their value since the disaster started, jumped in the last hour of Thursday trading on Wall Street after the oil stopped. But they were down again more than 3 percent Friday morning.
- Long after the well is finally plugged, oil could still be washing up in marshes and on beaches as tar balls or disc-shaped patties. The sheen will dissolve over time, scientists say, and the slick will convert to another form.
The Associated Press contributed to this report.