ON THE GULF OF MEXICO - BP embarked Tuesday on an operation that could seal the biggest offshore oil leak in U.S. history once and for all, forcing mud down the throat of its blown-out well in a tactic known variously as "bullheading" or a "static kill."

The pressure in the well dropped quickly in the first 90 minutes of the procedure, a sign that everything was going as planned, wellsite leader Bobby Bolton told The Associated Press aboard the Q4000, the vessel being used to pump in the mud. Hours later, Bolton told the AP that the procedure was still going well.

"Pressure is down and appears to be stabilizing," he said.

He said earlier that the work could be complete by Tuesday night or Wednesday, though BP said the effort could continue through Thursday, and engineers won't know for more than a week if it choked the well for good.

The 122 crew members on the Q4000 were excited about being part of what could be the final resolution to a drama that started with the April 20 explosion on the offshore drilling rig Deepwater Horizon, Capt. Keith Schultz said.

"I'm a mariner and we lost mariners out here," said Schultz, who is on his second 28-day tour of duty since the spill started. "I'm very confident we'll be able to kill this well. It's been one magical time trying to get this thing plugged."

A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but that is considered only a temporary measure. BP and the Coast Guard want to plug up the hole more securely with a column of heavy drilling mud and cement.

The static kill involves slowly pumping mud down lines running from a ship to the top of the ruptured well a mile below. BP said that may be enough by itself to seal the well.

But retired Coast Guard Adm. Thad Allen, the government's point man for the spill, made it clear that to be safe, the gusher will have to be plugged up from two directions. He said the 18,000-foot relief well that BP has been drilling over the past three months will be used later this month to execute a "bottom kill," in which mud and cement will be injected into the bedrock 2? miles below the sea floor.

"There should be no ambiguity about that," Allen said. "I'm the national incident commander and this is how this will be handled."

Over the past few months, with each failed attempt to stop the leak, the American public has learned some of the oil industry's lingo, including "top kill," which is similar to the static kill, "top hat," and "junk shot," an attempt to clog up the well with golf balls and rubber scraps.

Before the cap was lowered onto the well, 172 million gallons of crude flowed into the sea, unleashed by the April 20 explosion aboard the drilling rig Deepwater Horizon that killed 11 workers.

BP won't know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work.

Allen said the task is becoming more urgent because peak hurricane season is just around the corner. Tropical Storm Colin formed far out in the Atlantic on Tuesday, but early forecasts say it will travel toward the East Coast rather than the Gulf. And while the cap appears to be holding tight, the static kill would give scientists more confidence the well won't leak again, he said.

"The quicker we get this done, the quicker we can reduce the risk of some type of internal failure" of the massive cap, he said.

Gulf residents anxiously awaited the outcome. In Yscloskey, La., Russell Prats, a crab dealer, said he is confident the static kill will work, but concerned that people will still be scared to eat seafood.

"I think they'll be successful this time. I really do," he said. "But just because they kill the well doesn't mean our troubles go away."