A new study says balls of tar from last year's BP oil spill in the Gulf of Mexico off the southern United States are not breaking down as fast as officials had expected.

Auburn University researchers in Alabama say oil samples that washed up on the beach after a recent tropical storm appeared very similar to fresh oil deposits taken from the beach more than a year ago.

They say the data is contrary to the widely held belief that submerged oil which leaked for nearly three months from the Deepwater Horizon oil rig has “substantially” broken down.

BP has not commented on the new study. The company was out cleaning the beaches earlier this month after Tropical Storm Lee washed up remnants of the April 2010 spill.

The Deepwater Horizon oil rig exploded, killing 11 people and starting an 85-day underwater leak that spewed nearly 5 million barrels of oil into the Gulf of Mexico. It was the worst offshore oil spill in U.S. history.