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 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 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,” 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.”

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 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, coating birds and other wildlife and curtailing fishing.

“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 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.

Engineers planned to monitor the well overnight 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 Coast Guard Admiral Thad Allen, who is overseeing the operation. He added: “It’s a wait-and-see game here right now, so far nothing unfavorable.”

Meanwhile, dozens of witness statements obtained by The Associated Press show a combination of equipment failures and a deference to the chain of command impeded the system that should have stopped the gusher before it became an environmental disaster.

At a Coast Guard hearing that started earlier this month and continued in New Orleans on Wednesday, Doug Brown, chief rig mechanic aboard the platform, testified that the trouble began at a meeting hours before the blowout, with a “skirmish” between a BP official and rig workers who did not want to replace heavy drilling fluid in the well with saltwater.

The switch presumably would have allowed the company to remove the fluid and use it for another project, but the seawater would have provided less weight to counteract the surging pressure from the ocean depths.