(Watch live video of the blowout HERE.)
UPDATE: BP COO Doug Suttles tells Reuters that it appears that the oil gushing into the Gulf has been replaced by drilling mud. Suttles said that "...the way we know we've been successful is [oil] stops flowing.
COVINGTON, La. (AP) -- BP started pumping heavy mud into the leaking Gulf of Mexico well Wednesday and said everything was going as planned in the company's boldest attempt yet to plug the gusher that has spewed millions of gallons of oil over the last 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 know whether the attempt has been successful. The company wants to eventually inject cement into the well to seal it.
"I'm sure many of you have been watching the plume," Hayward said from Houston. "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."
The stakes are high. Fishermen, hotel and restaurant owners, politicians and residents along the coast are fed up with BP's so far ineffective attempts to stop the oil leak that sprang after an offshore drilling rig exploded April 20. Eleven workers were killed, and by the most conservative estimate, 7 million gallons of crude have spilled into the Gulf, fouling Louisiana's marshes and coating birds and other wildlife.
"We're doing everything we can to bring it to closure, and actually we're executing this top kill job as efficiently and effectively as we can," BP Chief Operating Officer Suttles said Wednesday night.
The top kill has worked above ground but has never before been tried 5,000 feet beneath the sea. Company officials peg its chance of success at 60 to 70 percent.
President Barack Obama said "there's no guarantees" it will work. The president planned a trip to Louisiana on Friday.
"We're going to bring every resource necessary to put a stop to this thing," he said.
Meanwhile, dozens of witness statements obtained by The Associated Press show a combination of equipment failure and a deference to the chain of command impeded the system that should have stopped the gusher before it became an environmental disaster.
A live video stream Wednesday showed pictures of the blowout preventer, as well as the 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 blow under the pressure, causing a brand new leak.
Gene Beck, a petroleum engineering professor at Texas A&M in College Station, said the endeavor would likely fail quickly if the mud could not overcome the pressure of the oil.
"The longer it goes, maybe the better news that is," Beck said.
Frustration with BP and the federal government has only grown since efforts to stop the leak have failed.
Louisiana Gov. 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 Coast Guard said.
Through the Mississippi's South Pass, there were miles-long passages that showed no indication of the oil, and the air smelled fresh and salty. Nearby fish were leaping and tiny seabirds dove into the water.
But not far away at Pass a Loutre, the odor wafting above the oily water was that of an auto shop.
"There's no wildlife in Pass a Loutre. It's all dead," Nungesser said.
The Coast Guard also said no dispersants were used Wednesday in an effort to reduce the chemicals in the Gulf, but crews were continuing the burn and skim the oil off the surface.