In a test, BP's new containment cap successfully contained the leak starting at 3:25 p.m. ET today, BP confirmed at an afternoon press conference. 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 on April 20th.
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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, issued a statement expressing cautious optimism.
"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.
While the test is going on, BP has disconnected two ships at the well site that were collecting oil, the Helix Producer and the Q4000. In their place, 40 skimmers are standing by to collect any oil that makes its way to the surface.
During the testing process, the company is also pausing its effort to drill relief wells. Engineers say they are the permanent way to plug the leak. The closer of the drill bits was, at last report, 4 feet away from the side of the blown-out well and 150 feet from the target depth where it would be turned to intercept oil from the leak.
Testing of the cap had been beset by delays since BP first lowered the device to the leak site Monday night.
First, government officials requested a delay for analysis, out of fears that the cap could damage the leaking well further and do more harm than good. After that 24-hour delay, the process began again but was delayed when BP found a leak in the line. It resumed this morning, resulting in this afternoon's apparent success.
On Wednesday, government officials defended the decision to delay the test.
"We did this to make sure that we were taking due care, and in some cases, maybe an overabundance of caution to make sure we didn't do any irreversible harm to the well," retired Coast Guard Adm. Thad Allen said at a news conference today. "It was advisable to take a 24-hour break to make sure we're getting it right."
Louisiana Gov. Bobby Jindal expressed support for the decision.
"We're at a point where we can't afford to make this worse," Jindal said. "We cannot afford for them to do damage to this well."
BP Stops Leaking Oil in Test
Concerns about the plan have centered on the piping inside the blown-out well that extends about 13,000 feet into the sea floor. If there are any weak spots in that pipe, high pressures could cause new leaks.
"They have to be very careful when they close it down," said Dr. Don Van Nieuwenhuise, a professor of geoscience at the University of Houston. "[They have to] make sure that pressure doesn't come up quickly and cause any of the pipes to rupture."
ABC News' Russell Goldman, Ned Potter and Zunaira Zaki contributed to this report.