Vessels assisting in the containment of oil from the Deepwater Horizon oil well leak are seen from the Pacific Responder oil skimming vessel Monday in the Gulf of Mexico near the coast of Louisiana.

A new containment cap is lowered over the broken wellhead at the site of the Deepwater Horizon oil spill Monday in the Gulf of Mexico in this image taken from video provided by BP. Deep-sea robots swarmed around BP’s ruptured oil well Monday in a delicately choreographed effort to attach the tighter-fitting cap that could finally stop crude from gushing into the Gulf of Mexico.

New Orleans ? BP robots attached a new, tighter-fitting cap on top of the gushing Gulf of Mexico oil leak Monday, raising hopes that the crude could be kept from polluting the water for the first time in nearly three months.

Placing the cap on top of the leak was the climax of two days of delicate preparation work and a day of slowly lowering it into position a mile below the sea. The capping project — akin to building an underwater Lego tower — is just a temporary fix, but the oil giant’s best hope for containing the spill.

The next unknown is whether the 18-foot-high, 150,000-pound metal stack of pipes and valves will work. BP plans to start tests today, gradually shutting the valves to see if the oil stops or if it starts leaking from another part of the well.

Residents have been skeptical BP can deliver on its promise to control the spill, but the news was still welcome on the coast. Dwayne Touchet, a 44-year-old shrimper from Welsh, La., said he was relieved to hear the cap was on and can only pray that it works. Touchet is working in the Vessels of Opportunity program, where BP employs local boat owners and fisherman out of work because of the spill.

“It’s not over, there’s still a lot of oil to clean up. We don’t know how it will affect it (the water) in the years to come, all we can do is trust in the Lord,” he said.

Around 6:30 p.m. CDT, live video streams trained on the wellhead showed the cap being slowly lowered into place, 11 hours after BP Chief Operating Officer Doug Suttles said the company was close to putting the seal in place. BP officials said the device was attached around 7 p.m.

The cap will be tested and monitored to see if it can withstand pressure from oil and gas starting this morning for six to 48 hours, according to National Incident Commander Thad Allen. On his Facebook page, Allen also shared news of the development. “Getting there,” Allen wrote in a status update shortly after the cap landed on the well.

The cap will be tested by closing off three separate valves that fit together snugly like pairs of fists, choking off the oil and blocking it from entering the Gulf.

BP doesn’t want the flow of oil to stop instantaneously, said Don Van Nieuwenhuise, director of Professional Geosciences Programs at the University of Houston. Shutting the oil off too quickly could cause another explosion, he said.

“Rather than like a train running into a brick wall, it’ll be more like putting the brakes on slowly,” he said. “That’s what they’re aiming for. You can keep the brakes on and everyone arrives alive, or you hit the wall and have big problems.”

Engineers will be watching pressure readings. High pressure is good, because it would mean the leak has been contained inside the wellhead machinery. But if readings are lower than expected, that could mean there is another leak elsewhere in the well.

“Another concern right now would be how much pressure the well can take,” and whether intense pressure would further damage the well, said Eric Smith, associate director of the Tulane Energy Institute.

Even if the cap works, the blown-out well will still be leaking. But the newer, tighter cap will enable BP to capture all the oil, or help funnel it up to ships on the surface if necessary.

One of those ships, the Helix Producer, began operating Monday and should be up to its capacity of collecting roughly 1 million gallons of oil a day within a few days, Suttles said.