BP will not know until tomorrow at the earliest if its complex attempt to plug the gushing Gulf of Mexico oil leak has worked.
And President Barack Obama warned there was no "silver bullet" solution to the biggest oil spill in US history.
Trying to assert leadership in the face of growing criticism, Obama toured the Louisiana coast yesterday, where oil has seeped into delicate marshlands and shut down much of the lucrative fishing trade. BP CEO Tony Hayward flew over the Gulf to where his crew and robots worked on the "top kill" -- the injection of heavy fluids, materials and ultimately cement to seal the well one mile below the surface.
Mr Hayward said the procedure was making progress choking off the five-week-old leak that has already spewed out millions of gallons of oil.
"We have wrestled it to the ground but we haven't put a bullet in its head yet," Mr Hayward said.
When the top kill began on Wednesday, BP said it would need up to 48 hours to gauge its success. But Mr Hayward extended the timeline another 24 to 48 hours yesterday.
He said the chance of success remained at 60 to 70pc.
BP has called the effort to plug the hole a "rollercoaster ride," and investors might say the same as BP shares lost 5pc.
On his second visit to the Gulf in the five-week crisis, Obama faced his own steep challenge to convince Americans that he was in command as frustrated Gulf Coast residents loudly criticised him for being slow to act and offering too little assistance.
"You will not be abandoned. You will not be left behind. We are on your side and we will see this through," Obama said.
"I am the president and the buck stops with me," he said.
The cost of the disaster so far is estimated at $930m (€750m). That figure is sure to multiply with clean-up of the oily mess.