COVINGTON, 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 during 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 attempt has been successful. The company eventually wants to inject cement into the well to permanently 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. Fisherman, 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 the Deepwater Horizon exploded on 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.

The "top kill" has worked above ground but never has 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" that it will work. He planned a trip to Louisiana on Friday.

"We're going to bring every resource necessary to put a stop to this thing," he said.

A live video stream Wednesday showed pictures of the well's 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 University, said the endeavor likely would 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 and testimony before a federal panel investigating the explosion show that 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.

In a handwritten statement to the Coast Guard obtained by the AP, Transocean rig worker Truitt Crawford said: "I overheard upper management talking saying that BP was taking shortcuts by displacing the well with saltwater instead of mud without sealing the well with cement plugs, this is why it blew out."

At a Coast Guard hearing Wednesday in New Orleans, Doug Brown, chief rig mechanic aboard the Deepwater Horizon 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.

Brown said the BP official, whom he identified only as the "company man," overruled the drillers.

Brown said the top official present for rig owner Transocean grumbled, "Well, I guess that's what we have those pinchers for," which he took to be a reference to devices on the blowout preventer, the five-story piece of equipment that can slam a well shut in an emergency.

Earlier that day, crews had finished injecting cement into the well to strengthen the sides and protect the pipe. At about 5 p.m., pressure tests revealed that something was wrong with the newly cemented well.

It passed one set of so-called positive pressure tests in which fluids were injected into the well to increase pressure to monitor whether the well remained stable. However, it failed a negative pressure test, in which fluid inside the well is reduced to see whether gas leaks into the well through the cement or casing.

Despite the pressure problems, BP decided at about 8 p.m. to resume extracting mud, but the pressure in part of the well spiked, gas escaped and the rig exploded.

The decision to proceed regardless may have been a "fundamental mistake," BP's own internal investigators told House investigators this week.

Oil spill glossary

Efforts to stop the Gulf Coast oil spill have given Americans a dizzying array of new technical terms to master. Among the least intuitive phrases:

Blowout preventer: A large valve at the top of an oil well that can be closed if a drilling crew loses control of it.

Acoustic trigger: A remote-controlled backup device for the blowout preventer, but one that was not used at Deepwater Horizon.

Containment dome: A giant metal box lowered onto the leak to corral the oil so it could be siphoned to the surface. It didn't work at Deepwater Horizon.

Top hat: A smaller version of the containment dome, it was considered for use at Deepwater Horizon, then abandoned in favor of other plans.

Top kill: Engineers armed with 50,000 barrels of dense mud and a fleet of robotic submarines attempt to plug the gusher a mile below the surface by pushing mud down through the blowout preventer into the well.