NEW ORLEANS -- The U.S. government said engineers will start work Monday to remove the temporary cap that stopped oil from gushing out of BP's blown-out Gulf well so that crews can raise a key piece of equipment from the seabed.

Retired Coast Guard Adm. Thad Allen, the government's point man for the spill response, told reporters Friday that engineers must remove the cap so they can raise the failed blowout preventer. The blowout preventer is considered a key piece of evidence in determining what caused the April rig explosion that unleashed the gushing oil.

The leak was first contained when engineers were able to place a cap atop BP's well. Workers then pumped mud and cement in through the top in a so-called "static kill" operation that significantly reduced pressure inside the well. Officials don't expect oil to leak into the sea again when the cap is removed, but Allen has ordered BP to be ready to collect any leaking crude just in case.

The Department of Justice and other federal investigators are overseeing the work to remove the blowout preventer, Allen said. The 50-foot (15-meter), 600,000-pound (272,000-kilogram) device -- which was designed to prevent such a catastrophe -- will be taken out of the water with the well pipe still inside to ensure the pipe doesn't break apart any more than it already has.

Keeping the blowout preventer intact is important because it's considered an essential piece of evidence in determining what caused the blast aboard the Deepwater Horizon on April 20. After the explosion, 206 million gallons (800 million liters) of oil spilled into the Gulf of

Mexico until the temporary cap stopped the flow. The explosion on the rig -- which was owned by Transocean Ltd. and being operated by BP PLC -- killed 11 workers.

Engineers are hoping the blowout preventer can be detached easily, but they are prepared to exert 80,000 pounds (36,000 kilograms) of pressure if needed, Allen said.

A new blowout preventer will be placed atop the well once the one that failed is raised. After that, the goal is to drill the final 50 feet (15 meters) of a relief well beginning Sept. 7, Allen said. From there, it will take about four days for drilling crews to reach their target.

The relief well has been called the ultimate solution to plugging the well. Once the relief well is drilled, engineers will be able to pump mud and cement in through the bottom of the well, plugging the one that gushed oil once and for all.

Meanwhile, the U.S. government said Friday that it is reopening more federal waters in the Gulf of Mexico for commercial and recreational fishing that had been closed because of the spill. The government is reopening 4,281 square miles (11,000 sq. kilometers) of waters off the coast of western Louisiana.

Oil sheen has not been seen there since July 29, and scientists found no oil or dispersants on samples of the area's shrimp and fish.

Twenty percent of federal waters in the Gulf remain closed.

"We're sort of nibbling at the edges if you will, areas that have been free of oil for the longest time and were oiled the least," Lubchenco said.

The news came as hearings continued in Houston before the joint U.S. Coast Guard-Bureau of Energy Management, Regulation and Enforcement investigative panel. On Friday, Mark Hafle -- a BP drilling engineer who was a key decision maker at the now-sunken rig -- exercised his constitutional right to refuse to testify.

The panel's goal is to determine what caused the explosion. The panel also will make recommendations to prevent such a catastrophe in the future.