There were reportedly cheers and slaps on the back in the BP offices as workers realized they'd finally stopped the leak after more than 200 millions of gallons of oil spewed into the ocean. Live video of the leak showed no oil at the site where plumes of oil had been seen billowing into the Gulf since the disaster began April 20.
At the White House, President Obama reacted to the news, saying, "I think it's a positive sign. We're still in the testing phase. I'll have more to say on it tomorrow."
Louisiana Gov. Bobby Jindal, a Republican, expressed cautious optimism in a statement.
"It is too early to declare victory. ..." Jindal said. "Our battles don't end even when the well is capped. Millions of gallons of oil are still in the Gulf, and some estimates show that oil will continue to hit our shores for many more months or maybe even longer."
BP is now conducting step-by-stop tests of the massive, 150,000 pound cap on the wellhead, a process the company said could take up to 48 hours. The company said it is fully possible that oil will escape again before the testing is done.
Engineers slowly ratcheted down the flow of oil this morning and afternoon, closing off three valves -- cutting the so-called kill line at 11:30 a.m. and then closing the choke line at 1:30 p.m., BP executive Kent Wells wrote on the company Twitter account.
Wells called it a "critical milestone," while stressing that it is not an assured success. Oil could flow again, based on the results of well-integrity tests, looking for leaks, that will last anywhere from six to 48 hours.
BP is monitoring pressure readings minute by minute, with higher pressures indicating a successful seal. Low pressures would indicate a leak.
If there is a leak, "they would have to start siphoning [the oil] off again to minimize the leak," Darryl Bourgoyne, the director of LSU's well research lab said today.
ABC News' Russell Goldman, Bradley Blackburn, Ned Potter and Zunaira Zaki contributed to this report.