An analysis of water, sediment and seafood samples taken in 2010 during and after the oil spill in the Gulf of Mexico has found higher contamination levels in some cases than previous studies by federal agencies did, casting doubt on some of the earlier sampling methods.

The lead author, Paul W. Sammarco of the Louisiana Universities Marine Consortium, said that dispersants used to break up the oil might have affected some of the samples. He said that the greater contamination called into question the timing of decisions by the National Oceanic and Atmospheric Administration to reopen gulf fisheries after the spill and that “it might be time to review the techniques that are used to determine” such reopenings.

Eleven workers died and roughly 200 million gallons of crude oil gushed into the gulf after a blowout at an exploratory well owned by BP caused the Deepwater Horizon drilling rig to explode on April 20, 2010. Nearly two million gallons of Corexit, a dispersant, were sprayed on the surface or injected into the oil plume near the wellhead.

In all, more than 88,000 square miles of federal waters were closed to commercial and recreational fishing. Some areas were reopened before the well was capped three months after the blowout; the last areas were reopened a year after the disaster.