Engineers monitoring BP Plc’s damaged well in the Gulf of Mexico detected seepage on the ocean floor that could mean problems with the cap that has stopped oil from gushing into the water, the government’s top oil spill official said on Sunday.

Earlier on Sunday, BP officials had expressed hope that the test of the cap which began Thursday could continue until a relief well can permanently seal the leak next month. Oil gushed from the deep-sea Macondo well for nearly three months until the new cap was put in place last week.

But late on Sunday, the U.S. government released a letter to BP Chief Managing Director Bob Dudley from retired Coast Guard Admiral Thad Allen that referred to an unspecified type of seepage near the mile-deep (1.6 km-deep) well along with “undetermined anomalies at the well head.”

“I direct you to provide me a written procedure for opening the choke valve as quickly as possible without damaging the well should hydrocarbon seepage near the well head be confirmed,” Allen wrote.