Robert, Louisiana (CNN) -- Three attempts to pump mud and 16 tries to stuff solid material into a breached Gulf of Mexico oil well failed to stop the flow, top BP executives said Saturday, and engineers and executives with the oil giant have decided to "move on to the next option."

That option: Place a custom-built cap to fit over the "lower marine riser package," BP chief operation officer Doug Suttles said. BP crews were already at work Saturday to ready the materials for that option, he said.

Suttles said three separate pumping efforts and 30,000 barrels of mud -- along with what chief executive officer Tony Hayward described as "16 different bridging material shots" -- just didn't do the trick.

"We have not been able to stop the flow," a somber Suttles told reporters. " ... Repeated pumping, we don't believe, will achieve success, so we will move on to the next option."

Suttles and other officials said that the "top kill" attempt to stop the flow did so -- but only as long as they were pumping. When the pumping stopped, the oil resumed its escape. And Coast Guard Rear Adm. Mary Landry said that BP would resume using undersea dispersants for the new attempt to trap the oil.

Suttles said the lower marine riser package cap "should be able to capture most of the oil" that has fed what is now the largest oil spill in U.S. history, but he cautioned that the new cap will not provide a "tight mechanical seal."

"We're confident the job will work, but obviously we cannot guarantee success at this time," he said.

Engineers should be ready in about four to seven days to make the fresh attempt, he said. Landry said officials were "disappointed in today's announcement," but noted that the immediate efforts to stop the flow were never intended to be permanent.

"The real solution, the end state, is a relief well," she said. BP currently is working on two relief wells, but they are not expected to be ready until August, Suttles said.

Earlier, Suttles said that BP engineers would try to place a second blowout preventer -- the piece of equipment that failed when the Deepwater Horizon exploded on April 20 -- should the lower marine riser package fail. The failed blowout preventer is a 48-foot-tall, 450-ton apparatus that sits atop the well 5,000 feet underwater.

Suttles and Landry praised the clean-up efforts, however, in light of the failure of the "top kill" attempt to stop the flow.

"It's a tribute to everybody that we only have 107 miles of shoreline oiled and only 32 acres of marsh," Landry said.

Meanwhile, teams in Louisiana were working Saturday on a clean-up project aimed at protecting coastal marshes. Plaquemines Parish President Billy Nungesser has said that machines would suck oil out of marshes Saturday after crews determined where to deploy them.

But Nungesser told CNN that BP needed to "step up to the plate tonight to save our wetlands" by using its might to create sand barriers to prevent the oil from moving into the marshes.

"BP needs to say it will pay to move those dredges and pump that sand berm," he said. "We are gonna die a slow death if we don't get that berm. We've got to have that barrier island."

President Barack Obama, who toured the area Friday, said federal officials were prepared to authorize moving forward with "a portion of" an idea proposed by local officials, who want the Army Corps of Engineers to build a "sand boom" offshore to keep the water from getting into the fragile marshlands.

But Nungesser said the marshes couldn't wait and that the effort needed to start immediately to save the Louisiana wetlands.

Government scientists on Thursday said as many as 19,000 barrels (798,000 gallons) of oil were spewing into the ocean every day, making this disaster perhaps twice the size of the Exxon Valdez incident.

Previously, BP officials and government scientists had said 5,000 barrels (210,000 gallons) of crude were flowing out daily.

"This is clearly an environmental catastrophe," Hayward said Friday. "There's no two ways about it."

In an e-mail message sent out after the announcement Saturday, Hayward said he was "disappointed that this operation didn't work."

"The team executed the operation perfectly, and the technology worked without a single hitch," he said. "We remain committed to doing everything we can to make this situation right."

Obama's visit to the region came under intense political pressure to take control of the situation.

"We want to stop the leak, we want to contain and clean up the oil and we want to help the people in this region return to their lives and livelihoods as soon as possible," the president told reporters.

About 25 percent of the Gulf of Mexico exclusive economic zone has been put off limits, according to the National Oceanic and Atmospheric Administration, and fishermen are worried the gushing oil will take a more serious toll than Hurricane Katrina did in 2005.

"Katrina was nothing but rain, water and wind. This is poison. It's gas," oysterman Arthur Etienne said.

CNN's Anderson Cooper contributed to this report.