BP said today everything was going as planned as the oil giant began pumping heavy mud into the leaking Gulf of Mexico well.
The "top kill" procedure is the company's boldest attempt yet to plug the gush that has spewed millions of gallons of oil over the past five weeks.
BP hoped the mud could overpower the steady stream of oil, but chief executive Tony Hayward said it would be at least 24 hours before officials knew whether the attempt worked. The company wants to eventually inject cement into the well to seal it.
"I'm sure many of you have been watching the plume," Mr Hayward said of the live video stream of the leak. "All I can say is it is unlikely to give us any real indication of what is going on. Either increases or decreases are not an indicator of either success or failure at this time."
Fishermen, hotel and restaurant owners, politicians and residents along the coast are fed up with BP's so-far ineffective attempts to stop the leak that sprang after an offshore drilling rig exploded on April 20.
Eleven workers were killed and, by the most conservative estimate, seven million gallons of crude have spilled into the Gulf, fouling Louisiana's marshes, coating birds and other wildlife and curtailing fishing.
"We're doing everything we can to bring it to closure, and we're executing this top kill job as efficiently and effectively as we can," BP chief operating officer Doug Suttles said today.
The top kill has worked above ground but has never before been tried 5,000 feet beneath the sea. Company experts give it a 60 to 70pc chance of success.
US president Barack Obama, who is due to visit Louisiana tomorrow, said "there's no guarantees" it would work.
"We're going to bring every resource necessary to put a stop to this thing," he said.
Engineers planned to monitor the well today and continue pumping in thousands of gallons of the drilling fluid, which is about twice as heavy as water.
"The absence of any news is good news," said US Coast Guard Admiral Thad Allen, who is overseeing the operation.
"It's a wait and see game here right now, so far nothing unfavourable."
Meanwhile, dozens of witness statements showed a combination of equipment failures and a deference to the chain of command impeded the system that should have stopped the gush before it became an environmental disaster.
The live video stream last night showed pictures of the blowout preventer and oil gushing out. At other times, the feed showed mud spewing out, but BP said this was not cause for alarm.
A weak spot in the blowout preventer could give way under the pressure, causing a brand new leak.
Frustration with BP and the government has only grown since efforts to stop the leak have failed.
Louisiana governor Bobby Jindal and Plaquemines Parish president Billy Nungesser, both outspoken critics, led a boat tour around the oil-fouled delta near the mouth of the Mississippi River. Some 100 miles of Louisiana coastline had been hit by the oil, the coastguard said.
BP has had some success in siphoning oil from a mile-long tube, which has sucked up 924,000 gallons of oil since it was installed last week. Engineers, though, had to move the device during the top kill.