BP has extended critical tests on its ruptured Gulf of Mexico oil well, despite for the moment no signs of any leaks.
The British firm remains cautiously optimistic it may have finally plugged the flow following the fitting of a new containment cap on Thursday.
But, pressure in the well is rising more slowly than hoped, suggesting there might be a leak elsewhere, allowing oil to seep up through the seabed.
Containment ships remain on stand-by to continue collecting the oil once tests finish later today.
Even if the cap proves successful, officials admit the only permanent fix is a relief well currently being dug.
The spill, the worst in US history, has ravaged hundreds of miles of Gulf coastline since April, creating an ecological and economic catastrophe.