An operation to permanently seal the ruptured oil well in the Gulf of Mexico has been delayed until early September, US officials say.
Coast Guard Admiral Thad Allen said oil firm BP needed to replace key equipment before starting the procedure, which involves pumping cement into the well.
BP had intended to start the process, known as "bottom kill", in mid-August.
The flow of oil was stopped on 15 July, but officials say the bottom kill is needed as a permanent fix.
Meanwhile, new research on the effects of the Deepwater Horizon spill has confirmed the presence of a toxic chemical residue 1km (0.6 miles) below the water's surface.
The delay will allow BP engineers to replace the failed blowout preventer - a giant stack of pipes and valves that sits atop the well.
Adm Allen refused to give a specific timescale for the operation, but said it would start probably in the week beginning 6 September.
"We will know when we have satisfied ourselves and we have removed any shadow of doubt," he said.
The investigation into the toxic residue, carried out in late June, confirms that a plume, or cloud, of crude oil exists below the surface.
The research, which is published in the journal Science, measures the plume of crude oil-based chemicals at up to 200m high and 2km wide, extending 35km from the spill site.
But the results of the investigation by a research team at the Woods Hole Oceanographic Institute, in the US state of Massachusetts, suggest that the quantity is less than 0.1% of the total amount spilled.
Members of the research team have said they are unable to speculate at this stage what impact the plume may have on marine life.
Oil began leaking into the Gulf on 20 April when the Deepwater Horizon rig exploded, killing 11 workers.
An estimated 4.9m barrels of oil leaked into the waters of the Gulf over the course of 87 days, with only 800,000 barrels being captured.
On 15 July, a cap was used to seal the top of the wellhead.