A rubber seal on the inside will attempt to keep oil from escaping, though engineers acknowledge some crude will still come out.
BP sliced off the main pipe on the leaking oil well with giant shears in the latest bid to curtail the worst oil spill in U.S. history, but the cut was jagged, and a looser fitting cap will be needed.
The placement of the cap was a positive step to contain the leak, but not a solution, said Coast Guard Adm. Thad Allen, the government's point man for the disaster.
"Even if successful, this is only a temporary and partial fix and we must continue our aggressive response operations at the source, on the surface and along the Gulf's precious coastline," Allen said in a statement.
BP PLC turned to the giant shears after a diamond-tipped saw became stuck in the pipe halfway through the job, yet another frustrating delay in the six-week-old spill.
If the cap can be put on successfully, BP will siphon the oil and gas to a tanker on the surface.
"It's an important milestone, and in some sense, it's just the beginning," BP CEO Tony Hayward said.
This latest attempt is risky because slicing away the section of the 20-inch-wide riser removed a kink in the pipe, and could temporarily increase the flow of oil by as much as 20 percent.
Live video footage showed oil spewing unimpeded from the top of the blowout preventer, but Allen said it was unclear whether the flow had increased.
"I don't think we'll know until the containment cap is seated on there," he said. "We'll have to wait and see."
Crews will also use methanol to try to prevent icylike crystals from forming on the inside of the cap. At this depth a mile underwater, the near-freezing temperatures can cause a buildup up of hydrates, which foiled the company's attempt to place a 100-ton, four-story dome over the leak about a month ago.
Meanwhile, newly disclosed internal Coast Guard documents from the day after the explosion aboard the Deepwater Horizon rig about 50 miles off the Louisiana coast indicated that U.S. officials were warning of a leak of 336,000 gallons per day of crude from the well in the event of a complete blowout.
The well didn't have such a failure. But the volume turned out to be much closer to that figure than the 42,000 gallons per day that BP first estimated. Weeks later that was revised to 210,000 gallons. Now, an estimated 500,000 to 1 million gallons of crude is believed to be leaking daily.
The Center for Public Integrity, which initially reported the Coast Guard logs, said it obtained them from Rep. Darrell Issa, R-Calif., ranking Republican on the House Oversight and Government Reform Committee.
White House spokesman Nick Shapiro said the president was worried about a leak much greater.
"Which is exactly why he ordered the entire federal government -- on the day the rig collapsed -- to treat this as the No. 1 priority and to devote every resource needed to respond to this incident and investigate its cause."
The logs also showed early in the disaster that remote underwater robots were unable to activate the rig's blowout preventer, which was supposed to shut off the flow from the well in the event of such a catastrophic failure.
BP has failed so far to plug the well.
The damage to the environment was chilling on East Grand Terre Island along the Louisiana coast, where workers found birds coated in thick, black goo. Images shot by an Associated Press photographer show Brown pelicans drenched in thick oil, struggling and flailing in the surf.
Anywhere between 21 million and 46 million gallons of oil has spewed into the Gulf, according to government estimates.
BP's Hayward promised Thursday that the company would clean up every drop of oil and "restore the shoreline to its original state."
"BP will be here for a very long time. We realize this is just the beginning," he said.