NEW ORLEANS — The oil has stopped. For now. After 85 days and up to 184 million gallons, BP finally gained control over one of America's biggest environmental catastrophes Thursday by placing a carefully fitted cap over a runaway geyser that has been gushing crude into the Gulf of Mexico since early spring. Though a temporary fix, the accomplishment was greeted with hope, high expectations — and, in many cases along the beleaguered coastline, disbelief. From one Gulf Coast resident came this: "Hallelujah." And from another: "I got to see it to believe it." If the cap holds, if the sea floor doesn't crack and if the relief wells being prepared are completed successfully, this could be the beginning of the end for the spill. But that's a lot of ifs, and no one was declaring any sort of victory beyond the moment. The oil stopped flowing at 3:25 p.m. EDT when the last of three valves in the 75-ton cap was slowly throttled shut. That set off a 48-hour watch period in which — much like the hours immediately after a surgery — the patient was in stable, guarded condition and being watched closely for complications. "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." Nevertheless, one comforting fact stood out: 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. 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."