NEW ORLEANS, La. -- Federal officials said Tuesday they have launched a criminal and civil probe into the nation's worst-ever oil spill, even as BP voiced hopes of capping the six-week-old Gulf of Mexico leak soon.

"We will prosecute to the fullest extent of the law anyone who has violated the law," Attorney General Eric Holder told reporters in New Orleans. "We will not rest until justice is done."

Holder said the criminal probe began "some weeks ago," but declined to elaborate what kind of charges could be brought and against whom. He was speaking after touring the region to witness first-hand the damage caused by the spill.

"What we saw this morning was oil for miles and miles. Oil that we know has already affected plant and animal life along the coast, and has impacted the lives and livelihoods of all too many in this region," he said. "This disaster is nothing less than a tragedy."

President Barack Obama earlier on Tuesday threatened to take legal action against those to blame, saying the government had an "obligation" to determine the cause of the "greatest environmental disaster of its kind in our history."

"If our laws were broken leading to this death and destruction, my solemn pledge is that we will bring those responsible to justice on behalf of the victims of this catastrophe and the people of the Gulf region," Obama said in a statement in the White House Rose Garden. "If the laws on our books are insufficient to prevent such a spill, the laws must change. If oversight was inadequate to enforce these laws, oversight has to be reformed."

Obama delivered his threat of legal action after meeting with former Florida senator Bob Graham and former EPA administrator William Reilly, who will co-chair a presidential commission into the Gulf of Mexico oil spill.

How's that stop-the-spill thing going? 

BP Chief Operating Officer Doug Suttles said the company now hoped to cap the rig's fractured pipe within the next 24 hours, thanks to a new operation launched Tuesday.

"If everything goes well, within the next 24 hours, we could have this contained," Suttles said in Louisiana.

BP shares plummeted Tuesday 15.49 percent following the news its "top kill" maneuver to drown the leak had failed on Saturday. It also revealed it had spent nearly a billion dollars trying to stop the leak and clean up the oil.

Tuesday's latest operation -- dubbed a lower marine riser pipe -- would cut off the jagged edges of the leaking pipe and seal it with a tight cap. A tube would then siphon most of the oil to a ship on the surface.

In cutting the riser pipe, however, BP runs the risk of unleashing an even larger torrent of oil.

If it works, BP engineers plan to lower another dome to capture a second flow of oil through a valve known as the blowout preventer, BP Managing Director Bob Dudley said on CNN. "By the end of the month, we are engineering a completely separate system that will make it more stormproof with a free-standing riser that would allow for quick disconnects if needed," he said.

Two deep relief wells being drilled by BP into the seabed to plug the leak permanently will not be ready until August.

An estimated 12,000 to 19,000 barrels of crude has been belching daily into Gulf waters since the explosion on the Deepwater Horizon rig, making for a total of more than 20 million gallons of oil.

BP has tried -- and failed -- several times in the past six weeks to cap the leak, triggering mounting anger as oil washes up on the Gulf shores, threatening rare animal and plant life.

Tuesday's official start of the hurricane season has worsened the outlook for residents in Louisiana and the neighboring states of Alabama and Mississippi, amid warnings the 2010 storm season will be more active than usual with up to 14 hurricanes.

Officials said so far 29 dead dolphins and 227 sea turtles had been collected in the area, which is above average for the time of year -- with at least one of each species having visible signs of oil.