The giant cap that's corked the oil flow in the Gulf of Mexico seems to be working, but the test won't be over until Sunday afternoon.

As the clock ran out on BP's 48-hour observation period Saturday, the U.S. official in charge of the spill clean-up, retired Gulf Coast Guard Admiral Thad Allen, said the "integrity test" would continue for another 24 hours.

That will be three days without the gush of oil that has been spewing since the Deepwater Horizon rig exploded on April 20, killing 11 workers.

BP senior vice president Kent Wells struck a cautiously optimistic tone Saturday.

"We're feeling more comfortable that we have integrity," Wells said at a briefing. "The test is not over. We haven't made final conclusions, yet. I want to keep stressing that, but at this point, there is no evidence that we don't have integrity."

After the well cap was attached Thursday, early tests showed pressure in the well was lower than expected. That sparked fears oil was escaping through some other unknown leak.

Now, clean up crews theorize the lower-than-expected pressure could be due to so much oil having already spilled out.

The University of Houston's Dr. Donald Van Nieuwenhuise says no matter the reason, the cap holding shows, "they have this under control."

"As long as it's holding the pressure that's building inside of it that's a good sign." he said.

After three months of unprecedented environmental and economic disaster, no one is celebrating just yet.

If the pressure reading were to drop suddenly, or another leak is discovered, the valves would be reopened, resuming the flow of oil into the Gulf of Mexico.

Allen said Saturday that when the well integrity testing is complete, the cap will be hooked up through pipes to ships on the surface that will collect the oil. That likely means more crude will be released into the Gulf, although it will be at a slower rate than before BP's latest fix.

Although the gushing oil may be contained for now, the spill continues to spread. That has many bracing for the inevitable aftermath and the ultimate cost of the cleanup.

BP has already pegged the cost of dealing with the disaster at more then $3.5 billion US.

Nevertheless, in the few short days since the gush of oil was stopped, there are already reports the crude is dissipating quickly.

Members of a Coast Guard crew that flew over the wellhead Saturday said far less oil was visible than just 24 hours earlier. Only a colorful sheen and a few long streams of rust-colored, weathered oil were apparent in an area that was covered by huge patches of black crude weeks earlier.

Somewhere between 94 million and 184 million gallons have spilled into the Gulf, according to U.S. government estimates.

The current cap is ultimately a temporary measure designed to stop the flow of oil until a relief well can be completed later this summer.

Prepared with files from CTV's Richard Madan and The Associated Press