NEW ORLEANS (AP) — Now the wait begins.

BP finally gained control over one of America’s biggest environmental catastrophes by placing a carefully fitted cap over a runaway geyser that has been gushing crude into the Gulf of Mexico since early spring. Engineers, politicians and Gulf residents will watch anxiously over the next day and a half to see if it holds.

After nearly three months and up to 184 million gallons, the accomplishment was greeted with hope, high expectations — and, in many cases along the beleaguered coastline, disbelief. But no one was declaring victory just yet.

“It’s a great sight,” said BP Chief Operating Officer Doug Suttles, who immediately urged caution. The flow, he said, could resume. “It’s far from the finish line. … It’s not the time to celebrate.”

Regardless, for the first time since an explosion on the BP-leased Deepwater Horizon oil rig killed 11 workers April 20 and unleashed the spill 5,000 feet beneath the water’s surface, no oil was flowing into the Gulf.

The next hours would be critical. Engineers and scientists would be monitoring the cap around the clock, looking for pressure changes. High pressure is good, because it shows there’s only a single leak. Low pressure, below 6,000 pounds per square inch or so, could mean more leaks farther down in the well.

President Barack Obama, who has encouraged, cajoled and outright ordered BP to stop the leak, called Thursday’s development “a positive sign.” But Obama, whose political standing has taken a hit because of the spill and accusations of government inaction, cautioned that “we’re still in the testing phase.”

The worst-case scenario would be if the oil forced down into the bedrock ruptured the seafloor irreparably. Leaks deep in the well bore might also be found, which would mean that oil would continue to flow into the Gulf. And there’s always the possibility of another explosion, either from too much pressure or from a previously unknown unstable piece of piping.

The drama that unfolded quietly in the darkness of deep water Thursday was a combination of trial, error, technology and luck. It came after weeks of repeated attempts to stop the oil — everything from robotics to different capping techniques to stuffing the hole with mud and golf balls.

The week leading up to the moment where the oil stopped was a series of fitful starts and setbacks.

Robotic submarines working deep in the ocean removed a busted piece of pipe last weekend, at which point oil flowed unimpeded into the water. That was followed by installation of a connector that sits atop the spewing well bore — and by Monday the 75-ton metal cap, a stack of lines and valves latched onto the busted well.

After that, engineers spent hours creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. They also shut down two ships collecting oil above the sea to get an accurate reading on the pressure in the cap.

As the oil flowed up to the cap, increasing the pressure, two valves were shut off like light switches, and the third dialed down on a dimmer switch until it too was choked off.

And just like that, the oil stopped.

It’s not clear yet whether the oil will remain bottled in the cap, or whether BP will choose to use the new device to funnel the crude into four ships on the surface.

For nearly two months, the world’s window into the disaster has been through a battery of BP cameras, known as the “spillcam.” The constant stream of spewing oil became a fixture on cable TV news and web feeds.

That made it all the more dramatic on Thursday when, suddenly, it was no more.

On the video feed, the violently churning cloud of oil and gas coming out of a narrow tube thinned, and tapered off. Suddenly, there were a few puffs of oil, surrounded by cloudy dispersant that BP was pumping on top. Then there was nothing.

“Finally!” said Renee Brown, a school guidance counselor visiting Pensacola Beach, Fla., from London, Ky. “Honestly, I’m surprised that they haven’t been able to do something sooner, though.”

Alabama Gov. Bob Riley’s face lit up when he heard the news. “I think a lot of prayers were answered today,” he said.

Thad Allen, the retired Coast Guard admiral overseeing the spill for the government, said they are deciding as they go along whether to release oil into the water again. At the end of the 48-hour test it’s possible oil will start to flow again — but, theoretically, in a controlled manner.

When the test is complete, more seafloor mapping will be done to detect any damage or deep-water leaks.

The saga has devastated BP, costing it billions in everything from cleanup to repair efforts to plunging stock prices. Though BP shares have edged upward, they shot higher in the last hour of trading on Wall Street after the company announced the oil had stopped. Shares rose $2.74, or 7.6 percent, to close at $38.92 — still well below the $60.48 they fetched before the rig explosion.

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, by fate even — was evident in the wide spectrum of reactions to news of the capping.

“Hallelujah! That’s wonderful news,” Belinda Griffin, who owns a charter fishing lodge in Lafitte, La., said upon hearing the gusher had stopped. “Now if we can just figure out what to do with all the oil that’s in the Gulf, we’ll be in good shape.”

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, which helps explain why one oysterman in Louisiana refused to accept that progress was afoot.

At Get-Away-Lodge in Plaquemines Parish, the worst-hit area of the coast, three fishing captains changed oil in the boats they once used for fishing, but are a part of BP’s vessel of opportunity program when they heard.

They were pleased, but concerned — and worried about how long their jobs for BP will last.

“I think it’s wonderful they capped it, but it’s not helping our businesses,” said Chad Horton, 32, a native of Buras, who used to make a living putting customers on schools of redfish and speckled in these bountiful waters. “Our businesses are gone, but we’re depending on this (BP job) to support our families. They could come in and pull it out from under us at any time.”

Rosalie Lapeyrouse, who owns a grocery store and a shrimping operation in Chauvin, La. that cleans, boils and distributes the catch, was shocked.

“It what?” she said in disbelief. “It stopped?” she repeated after hearing the news.

“Oh, wow! That’s good,” she said, her face clouding. “I’m thinking they just stopped for a while. I don’t think it’s gonna last. They never could do nothing with it before.”

Long after the out-of-control 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.

There’s also fear that months from now, oil could move far west to Corpus Christi, Texas, or farther east and hitch a ride on the loop current, possibly showing up as tar balls in Miami or North Carolina’s Outer Banks.

The National Oceanic and Atmospheric Administration is expecting to track the oil in all its formations for several months after the well is killed, said Steve Lehmann, a scientific support coordinator for the federal agency.

Once the well stops actively spewing oil, the slicks will rapidly weather and disappear, possibly within a week, and NOAA will begin to rely more heavily on low-flying aircraft to search for tar balls and patties. Those can last for years, Lehmann said.

In St. Bernard Parish, oysterman Johnny Schneider stood near his boat, loaded not with seafood, but yellow plastic boom used to contain oil on the water.

“Eh, the damage is done. The oil’s everywhere now,” he said. “You ain’t never gonna get it out of the water.”

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Weber reported from Houston. Associated Press Writers Shelia Byrd, Jay Reeves, Mary Foster, Alan Sayre, Kevin McGill, Jennifer Garske King, Matt Sedensky, Pauline Arrillaga and Ramit Plushnick-Masti contributed to this report.