(CNN) -- Researchers have doubled estimates of how much oil has been spewing from a ruptured well in the Gulf of Mexico, reporting Thursday that up to 40,000 barrels (1.7 million gallons) a day may have escaped for weeks.
Well owner BP has been able to capture a varying percentage of that oil, first with a siphon inserted into the well riser and since June 3 with a cap that allowed workers to draw nearly 16,000 barrels to a ship on the surface Wednesday.
But scientists from the Flow Rate Technical Group reported Thursday that the amount of oil that has been leaking into the Gulf since late April was roughly twice the amount they previously estimated.
"The lowest estimate that we're seeing, that the scientists think is credible, is probably about 20,000 barrels," U.S. Geological Survey Director Marcia McNutt said. "And the highest that we're seeing is probably a little over 40,000, maybe a little more, depending on whether there are any systematic issues with gas."
Two weeks ago, researchers put the amount of oil escaping into the Gulf at between 12,000 and 19,000 barrels (504,000 to 798,000 gallons) per day.
Scientists estimate that the spill's flow rate increased by 4 to 5 percent after the well's riser pipe was cut last week in order to place the cap atop the well.
In a statement Thursday, BP said the company "fully supported this effort, providing the scientific team with data, including a considerable amount of high-resolution video."
The latest figure was calculated in part by using high-definition video of the spill that BP released this week after demands from members of Congress.
Still, Rep. Edward Markey, D-Massachusetts, said there will be an opportunity in a few weeks for the scientists to get a better estimate of the size of the flow. The group has told BP that they could get a more accurate estimate if they can send down equipment to measure the flow as the company attempts to swap out a the containment cap on the well.
BP has yet to agree to the request, Markey noted in a letter to the company Thursday. "There are concerns that, without the best information on the size and force of this gusher, that the effectiveness of the new containment cap and relief wells could be compromised," the letter says.
On Monday, BP announced that the new cap would fit better and serve as a better long-term containment option "to provide the greatest flexibility for operations during a hurricane and (it) is expected to be implemented in early July."
The measurements are crucial to analyzing the flow, said Ira Liefer, a researcher at the Marine Sciences Institute at the University of California at Santa Barbara. "So, instead of doing a very difficult effort to figure out how much oil and gas is coming up, we can measure it and know."
BP confirmed receiving Markey's letter and would only say, "We plan to respond in due course."
Scientists from the Geological Survey, the Department of Energy and the National Oceanic and Atmospheric Administration and experts from universities and other research institutions worked on the latest estimates, McNutt said. The information is expected to help guide efforts to stop the spill and deal with its impact on coastal areas, fish and wildlife.
The estimates came from several research teams using a variety of methods to reach their conclusions, said McNutt, who led the technical group calculating the size of the spill. But the analysis "is still a work in progress," she said.
The issue has been particularly challenging because the leak is occurring a mile below the water surface.
"Each of these methods has slightly different systematic biases in them," she said. "We understand that, as scientists, so that doesn't worry us."
She said the exercise will help scientists in any future spills.
"We will learn so much more about measuring oil in the ocean that we will be able to do a much better job next time in terms of how we go about measuring the release of oil and the inventory of oil in the ocean."