NEW ORLEANS – A tightly fitted cap was successfully keeping oil at a busted well from gushing into the Gulf of Mexico for the first time in three months, BP said.

Now begins a tense 48 hours of watching to see whether the capped-off well will hold or blow a new leak.

To the relief of millions of people along the Gulf Coast, the big, billowing brown cloud of crude at the bottom of the sea disappeared from the underwater video feed for the first time since the disaster began in April, as BP closed the last of three openings in the 75-ton cap lowered onto the well earlier this week.

But the company stopped far short of declaring victory over the biggest offshore oil spill in US history and one of the nation’s worst environmental disasters, a catastrophe that has killed wildlife and threatened the livelihoods of fishermen, restaurateurs, and oil industry workers from Texas to Florida.