NEW ORLEANS - Engineers will soon start the delicate work of detaching the temporary cap that stopped oil from gushing from BP's blown-out Gulf of Mexico well and the hulking device that failed to prevent the leak - all while trying to avoid more damage to the environment.

Retired Coast Guard Adm. Thad Allen, the government's point man for the spill response, told reporters Friday that engineers will remove the cap starting Monday 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, 600,000-pound 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 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.

Work to lift the blowout preventer is extremely difficult and delicate - all happening a mile underwater. Engineers must take care not to raise the central casing of the well and a casing seal. They also may have to carefully free the blowout preventer from any hanging drill pipe.

Raising the device may require as much as 80,000 pounds of pressure, 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 of a relief well beginning Sept. 7, which will take about four days, Allen said.

The relief well has been called the ultimate solution to plugging the well that blew out. Once the relief well is drilled, engineers will be able to pump in mud and cement to permanently plug the well that gushed oil.

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 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 finfish.

Twenty percent of federal waters in the Gulf remain closed.

The news came as the joint U.S. Coast Guard-Bureau of Energy Management, Regulation and Enforcement investigative panel wrapped up five days of hearings. 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. It will reconvene in October and hold one more session after that.