New Orleans ? In a nail-biting day across the Gulf Coast, engineers struggled to make sense of puzzling pressure readings from the bottom of the sea Friday to determine whether BP’s capped oil well was holding tight.

Halfway through a critical 48-hour window, the signs were promising but far from conclusive.

Kent Wells, a BP PLC vice president, said on an evening conference call that engineers had found no indication that the well has started leaking underground.

“No news is good news, I guess that’s how I’d say it,” Wells said.

Engineers are keeping watch over the well for a two-day period in a scientific, round-the-clock vigil to see if the well’s temporary cap is strong enough to hold back the oil, or if there are leaks either in the well itself or the sea floor. One mysterious development was that the pressure readings were not rising as high as expected, said retired Coast Guard Adm. Thad Allen, the government’s point man on the crisis.

Allen said two possible reasons were being debated by scientists: The reservoir that is the source of the oil could be running lower three months into the spill. Or there could be an undiscovered leak somewhere down in the well. Allen ordered further study but remained confident.

“This is generally good news,” he said. But he cautioned, “We need to be careful not to do any harm or create a situation that cannot be reversed.”

He said the testing would 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.

Throughout the day, no one was declaring victory — or failure. President Barack Obama cautioned the public “not to get too far ahead of ourselves,” warning of the danger of new leaks “that could be even more catastrophic.”

Even if the cap passes the test, more uncertainties lie ahead: Where will the oil already spilled go? How long will it take to clean up the coast? What will happen to the region’s fishermen? And will life on the Gulf Coast ever be the same again?

“I’m happy the well is shut off, that there’s a light at the end of the tunnel,” said Tony Kennon, mayor of hard-hit Orange Beach, Ala. But “I’m watching people moving away, people losing their jobs, everything they’ve got. How can I be that happy when that’s happening to my neighbor?”

On Thursday, BP closed the vents on the new, tight-fitting cap and finally stopped crude from spewing into the Gulf of Mexico for the first time since the April 20 oil-rig explosion that killed 11 workers and unleashed the spill 5,000 feet down.

With the cap working like a giant cork to keep the oil inside the well, scientists kept watch on screens at sea and at BP’s Houston headquarters, in case the buildup of pressure underground caused new leaks in the well pipe and in the surrounding bedrock that could make the disaster even worse.

Pressure readings after 24 hours were about 6,700 pounds per square inch and rising slowly, Allen said, below the 7,500 psi that would clearly show the well was not leaking. He said pressure continued to rise between 2 and 10 psi per hour. A low pressure reading, or a falling one, could mean the oil is escaping.

But Allen said a seismic probe of the surrounding sea floor found no sign of a leak in the ground.

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.

“If it’s rising slowly, that means the pipe’s integrity’s still there. It’s just getting around obstacles,” he said. He added that “any increase in pressure is good, not bad.”

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.