Images from the Gulf of Mexico suggest a once vast expanse of oil is breaking up so rapidly it may soon be invisible to satellite photography. But scientists warned today that underwater plumes of oil could linger for a year or even decades.

One hundred days after the explosion aboard the Deepwater Horizon, the US moved into a new phase in its response to the country's worst environmental disaster today.

John Amos, president of SkyTruth, an environmental satellite organisation, said the slick was "breaking up in more isolated patches. In the next few days, if there are no new oil leaks, we expect those patches to break down so that we can't see them in satellite images."

Amid the relatively good news about the slick, the justice department has stepped up its criminal investigation of BP and two other companies and is assembling a "BP squad" in New Orleans, the Washington Post reported.

In Washington, House and Senate Democrats have introduced bills to toughen government oversight of offshore drilling and make oil companies more responsible for damage caused by spills.

The House version of the bill could see BP shut out of future offshore drilling projects in the US, with a proposed ban on new drilling for oil companies that have had more than 10 deaths offshore. The Senate bill came as a huge disappointment to businesses and environmental organisations, which had hoped the spill would give a boost to climate change legislation. It provides only a token bow to climate change in incentives for electric cars.

In the Gulf, SkyTruth, which had warned early on that the spill was far greater than BP's estimates, said the total area covered by the oil slick was significantly reduced.

"It appears to be on its way out – at least the stuff we can see floating on the surface, " said Amos. "We don't see any obvious new oil coming to the surface at the site of the well and that is a good sign. We think what we are seeking is residual oil slick that is steadily breaking up, being collected or being dispersed naturally by evaporation."

After several failed attempts, BP capped the well on 15 July by installing a new, tighter-fitting cap. The company says, though, that the leak will not be stopped for good until a relief well is completed next month.

But scientists said it was unclear what was happening in the ocean depths and warned that oil could already be buried in coastal marshes. Tar balls continued to wash up on the coast of Louisiana this week.

"Less oil on the surface does not mean that there isn't oil beneath the surface, however, or that our beaches and marshes are not still at risk," Jane Lubchenco, the head of the National Oceanic and Atmospheric Administration, told reporters.

Scientists are worried that most of the oil remains trapped below the surface by the nearly 800,000 gallons of chemical dispersants that were pumped into the ocean depths.

John Kessler, an oceanographer at Texas A&M university who led a research expedition to the Gulf last month, said the experience of natural releases of oil and natural gas suggested the oil would remain in the deep water long after it had disappeared from the surface. "The oil could remain for anywhere from a year up to decades," he said.

He detected thick underwater plumes of oil from just below the surface to depths of 3,000ft within a 10-mile radius of BP's ruptured well. "It is most likely that this plume of natural gas and oil is not going to immediately dissipate, even if there is no other source in the water," he said.