New Orleans, Louisiana (CNN) -- A mechanical problem prevented BP from inserting a tube into a ruptured pipe that would help siphon off oil leaking into the Gulf of Mexico, BP official Doug Suttles said Saturday.
Suttles said the device was hoisted back to the surface Friday for readjustments and the company expects to have it working by Saturday night. The plan is for the mile-long tube to collect the oil and send it to a surface vessel.
Millions of gallons of crude have gushed into the Gulf since the fiery explosion and sinking of the Deepwater Horizon drilling rig more than three weeks ago.
Suttles, BP's chief operating officer for exploration and production, cautioned that the method is designed to contain the flow, not stop it.
The tactic was dealt a setback Friday night when the frame holding the insertion tube shifted and prevented the surface vessel from connecting to it, Suttles said.
Meanwhile, Suttles said the application of underwater dispersants -- a tactic approved for use Friday -- "appears" to be working.
"The oil in the immediate vicinity of the well and the ships and rigs working in the area is diminished from previous observations," he said after flying over the area Saturday.
He did not provide further details.
The Coast Guard and the Environmental Protection Agency said Friday that the decision to use subsea dispersants is an "important step" at reducing potential damage from the spill, because dispersant can be more effective underwater than on the ocean's surface.
Oil dispersants are chemicals that can break the oil down into small drops and prevent it from reaching the surface or the shore. Dispersants are generally less harmful than the oil itself, which is highly toxic, and they biodegrade more quickly, the Coast Guard said.
The underwater gusher began after an April 20 explosion aboard the Deepwater Horizon. The explosion and subsequent fire caused the Deepwater Horizon to sink two days later, prompting oil to begin spilling from the well. BP was leasing the rig from Transocean.
On Saturday, the Department of Homeland Security released a letter from Homeland Security Secretary Janet Napolitano and Interior Secretary Ken Salazar addressed to BPchief executive Anthony Hayward, calling on the company to state its "true intentions" for compensating those affected by the spill.
"The BP Deepwater Horizon oil spill may prove to be one of the most devastating environmental disasters this nation has ever faced," said the letter, which was dated Friday. "As one of the responsible parties for this event, BP is accountable to the American public for the full clean-up of this spill and all the economic loss caused by the spill and related events."
The letter also asks BP to confirm it will not "seek reimbursement from the American taxpayers, the United States government or the Oil Spill Liability Trust Fund for any amount."
There was no immediate response from BP.