WASHINGTON -- The US Justice Department is still investigating the causes of the Gulf of Mexico oil spill to determine whether to bring criminal charges, Attorney General Eric Holder said Sunday.

"The investigation is ongoing. We are in the process of accumulating documents, talking to witnesses on both the criminal side and the civil side," he told CBS "Face the Nation."

But Holder said there was no timetable to decide whether charges would be brought against BP, which leased the Deepwater Horizon rig from Transocean, the world's largest offshore drilling contractor based in Houston, Texas.

The rig exploded on April 20 killing 11 workers and then sank two days later, unleashing the nation's worst ever environmental disaster with tens of thousands of barrels of crude gushing into the sea every day.

"Our primary concern at this point is getting the spill stopped," Holder said, as the crisis entered it 13th week and BP engineers raced to install a new cap the British energy giant hopes will contain all the leaking crude.

A system installed by robotic submarines a mile down on the sea floor has been siphoning up oil to surface vessels for the past few weeks, but the slick has washed ashore in four southern US states, devastating industries such as fishing and tourism.

Holder was quick to stress that when he announced the probe on June 1, he had been careful not to mention BP by name as it was not the only party involved with the Deepwater Horizon rig.

"What I did say was that we had opened a criminal investigation but did not indicate who the subject of the investigation was," Holder said.

"And that is a very serious thing because there are a variety of entities and a variety of people who are the subjects of that investigation. And for people to conclude that BP is the focus of this investigation might not be correct."

At congressional hearings back in May, BP, Transocean and Halliburton blamed each other for the spill as executives from all three oil titans were grilled by US lawmakers.

BP says rig owner Transocean was responsible for the failure of the giant blowout preventer valve which made it impossible to regain control of the well, but Transocean said all the operations were run by BP.

The finger was also pointed at Halliburton, the oil services company which was responsible for vital cement work around the wellhead, which should have sealed the exploratory well until full production began.