BP failed again Saturday to plug a runaway well in the Gulf of Mexico with heavy drilling mud and cement, marking another setback in its efforts to stop the worst oil spill in American history. After pumping mud into the damaged well shaft for three days, the London-based oil giant conceded that the procedure, known as a 'top kill,' was unlikely to work in the mile-deep waters off the Louisiana coast. 'This scares everybody, the fact that we can't make this well stop flowing, the fact that we haven't succeeded so far,' said Doug Suttles, BP's chief operating officer. 'Many of the things we're trying have been done on the surface before, but have never been tried at 5,000 feet.' BP's next option involves cutting and removing the leaking riser at the top of the blowout preventer, the five-story contraption above the wellhead. Engineers will then install a cap that would be connected to a new pipe from a drillship on the surface. Suttles said he is hopeful the procedure would capture most of the oil escaping from the well. It should take four to seven days to complete the maneuver. At the same time, BP is continuing to drill a relief well, which experts say is the surest bet to stop the flow, but may take until August to finish. After the announcement, President Barack Obama said in a statement that ongoing flow of oil is 'as enraging as it is heartbreaking.' Obama said the new plan to capture the oil is risky, but 'we will continue to pursue any and all responsible means of stopping this leak.' The oil is gushing from BP's Macondo well, which blew out April 20, toppling the Deepwater Horizon rig and killing 11 workers. The ruptured well has spilled between 18 million and 40 million gallons of oil into the Gulf over the past five weeks, fouling more than 100 miles of coastline, based on official estimates. The ecological toll of the spill thus far includes 478 birds, 224 sea turtles and 25 marine mammals, according to the latest federal tally. But officials cautioned that tests are ongoing to determine how many died from oil exposure. The failure of the top kill procedure was perhaps the toughest blow in the series of unprecedented steps to stop the leak. Some experts said it was BP's best option to end the slow-motion saga before the completion of the relief well. 'We're all very disappointed,' Coast Guard Rear Adm. Mary Landry said. 'All of us want to see this well capped.' Aaron Viles of the Gulf Restoration Network, a New Orleans-based environmental group, said the latest failure 'packs more of a punch.' 'It shows that there was no acceptable or appropriate plan B,' Viles said. 'Offshore drilling is a high-wire act without a net, and we're all paying the price for it.' BP officials decided to scrap the delicate procedure after injecting about 30,000 barrels of drilling fluids, or mud, into the out-of-control well to help create pressures great enough to stanch the flow. The company also made three attempts at the 'junk shot,' which involves force-feeding chunks of rubber, golf balls and other materials with the mud into the well. Suttles said the efforts were unable to consistently hold back the oil and thus prevented the capping of the damaged well with cement, but it's unclear why. The new plan will not kill the well but is intended to capture most of the oil. 'I want to stress the word most, because it's not a tight, mechanical seal,' Suttles said. Suttles said he is confident that the procedure will work. But it could have the reverse effect of creating a bigger opening for oil to escape if the installation of a new valve goes awry, experts said. 'If they can't get that valve on, things will get much worse,' Philip W. Johnson, an engineering professor at the University of Alabama, told the Associated Press. Johnson said he thinks the plan can succeed, but added: 'It's a scary proposition.' Meanwhile, Coast Guard and Minerals Management Service officials concluded a sixth day of testimony during hearings into the disaster in Kenner, La. The hearings are scheduled to resume in July. Coast Guard Capt. Hung Nguyen, chairman of the joint investigative panel, said two senior officers on the ill-fated rig -- Capt. Curt Kuchta and Jimmy Harrell, the offshore installation manager -- are now 'parties of interest' in the probe because of their 'conduct.' Other parties of interest are BP, Transocean and some BP contractors. matthew.tresaugue@chron.com XXX - End of Story3D 