ROBERT, La. — 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 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 know whether the “top kill” 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 ineffective attempts to stop the oil leak that began 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 Doug Suttles said Wednesday night.

The top-kill method has worked above ground but has never been tried 5,000 feet beneath the sea. Company officials estimate its chance of success at 60 to 70 percent.

President 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.

“The absence of any news is good news,” said Coast Guard Adm. Thad Allen, who is overseeing the operation. “It’s a wait-and-see game here right now, so far nothing unfavorable.”

A live video stream Wednesday showed pictures of the blowout preventer, as well as oil gushing out. At other times the video showed mud spewing, but BP said this was not a cause for alarm.

A weak spot in the blowout preventer could give way under the pressure, causing a new leak.

Gene Beck, a petroleum engineering professor at Texas A&M University, 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.

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 oil before it became an environmental disaster.

Frustration with BP and the federal government has continued to grow as 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.

About 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 oil, and the air smelled fresh and salty. 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.

BP has had some success in siphoning oil at the source 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.

Crews are continuing to burn and skim the oil off the surface.

Engineers are working on backup plans in case the top kill doesn’t work, including a bid to cap the well with a small containment dome.

Suttles was trying to temper expectations Wednesday. He said it’s too early to express optimism about the top kill.

“It’s too hard to say. We’ve all been here a long time,” he said. “We’ve ridden a roller coaster and we need to take the next 24 hours and see what the results are.”