In May 2010, McNutt headed the Flow Rate Technical Group which attempted to measure the Deepwater Horizon oil spill in the Gulf of Mexico. Preliminary reports from the group said that the rate of the oil spill was at least twice and possibly up to five times as much as previously acknowledged. Subsequent estimates, based on six independent methodologies, were four times the 1989 Exxon Valdez oil spill. A refined estimate based on new pressure readings, data, and analysis, released by the United States Secretary of Energy Steven Chu and McNutt in August, said that 4.9 million barrels (with uncertainty of plus or minus approximately 10 percent) of oil had leaked from the well until it was capped on July 15. The disaster was the largest ever accidental spill of oil into marine waters.