(CNN) -- BP on Saturday was conducting a "fishing" operation aimed at finding and removing up to 3,500 feet of drill pipe from the capped Gulf well.
"[The pipe] is more than likely in several pieces," said BP spokesman Bill Salvin, who said the procedure will take at least two days.
The federal government authorized the removal of the pipe before the well's failed blowout preventer is replaced with a new one. The pipe ran from the Deepwater Horizon oil rig, through the blowout preventer and into the well.
Work was progressing Saturday afternoon, Salvin said.
A 48-hour ambient pressure test on whether the pressure in the top of the well matches the pressure outside the well was a success, BP said.
If all goes as planned and the blowout preventer is successfully replaced, the "bottom kill" operation to permanently plug the ruptured underwater well in the Gulf of Mexico could be completed by the week after Labor Day, he said.
Government response commander Thad Allen authorized BP to present a blowout preventer removal plan by Sunday.
"The procedure should also include measures to enable capture and removal of oil in the event of a release during the procedure," Allen wrote.
That removal is under close supervision by government scientists, BP engineers and joint investigation teams -- all of whom will want to examine the device closely to gain insight into what happened during the explosion on April 20 when the Deepwater Horizon exploded, killing 11 workers, and later sank.
In a letter Friday night, Allen instructed BP to preserve the old blowout preventer, capping stack and other equipment so that federal investigators can look at them.
The government estimates some 4.9 million barrels of oil gushed into the Gulf, 800,000 of which was captured by surface ships.
The well was capped July 15, stopping the flow of oil, and a "static kill" operation two weeks ago further plugged it with cement and mud from above. The "bottom kill" operation is believed to be a permanent fix, in which the well will be intercepted by a relief well and the ruptured well will be plugged from below.
CNN's Vivian Kuo and Phil Gast contributed to this report.