U.S. researchers said Thursday that 4.4 million barrels of oil spilled into the Gulf of Mexico after a deep water well ruptured in April. The figure represents the first independent peer-reviewed study of the worst oil spill in U.S. history and conforms closely to the most recent estimate by government scientists.

Since the spill, official estimates of how much oil was released kept changing, from 1,000 barrels a day to 5,000, then to 12,000, then 19,000, and the numbers continue to rise.

Using a new technique to analyze underwater video of the well riser, they say it leaked some 56,000 to 68,000 barrels daily -- maybe more -- until the first effective cap was installed, on July 15. Their estimate of the total oil escaped into the open ocean is some 4.4 million barrels -- close to the most recent consensus of government advisors, whose methods have not been detailed publicly.

"We wanted to do an independent estimate because people had the sense that the numbers out there were not necessarily accurate," said lead author Timothy Crone, a marine geophysicist at Columbia University's Lamont-Doherty Earth Observatory.

After BP and government officials downplayed the possibility or importance of measurements, a wide spectrum of scientists, environmental groups and legal experts pointed out that the information was needed to determine both short- and long-term responses, and monetary liability.

The new study divides the flow rate into two periods: April 22 to June 3, when oil spurted from a jagged break in the riser; and after June 3, when the riser was cut, and oil temporarily spewed into the ocean unimpeded.

Crone and his coauthor, Lamont marine geophysicist Maya Tolstoy, used a visual analysis technique Crone recently developed called optical plume velocimetry. They say video from the earlier period indicated a flow of about 56,000 barrels a day (a barrel is 42 gallons). After the pipe was cut, they say, the rate went to about 68,000. After accounting for time elapsed, the authors subtracted 804,877 barrels collected by BP at the site, to come up with a total of 4.4 million barrels that escaped. Given the study's stated 20 percent margin of error plus or minus, this roughly agrees with the federal government's Flow Rate Technical Group's most recent comparable estimate of 4.1 million barrels (after subtracting the oil collected by BP).

The study appears Thursday in the online edition of the leading journal Science.