(Reuters) - Millions of gallons/liters of oil have spewed into the Gulf of Mexico since an April 20 explosion on a drilling rig triggered the worst spill in U.S. history.

Below is a timeline of the disaster and its impact.

April 20, 2010 - Explosion and fire on Transocean Ltd’s drilling rig Deepwater Horizon licensed to BP Plc; 11 workers are killed. The rig was drilling in BP’s Macondo project 42 miles southeast of Venice, Louisiana, beneath about 5,000 feet of water and 13,000 feet under the seabed.

April 22 - The Deepwater Horizon rig, valued at more than $560 million, sinks and a 5-mile oil slick forms.

April 25 - Efforts to activate the well’s blowout preventer fail.

April 29 - U.S. President Barack Obama pledges “every single available resource,” including the U.S. military, to contain the spreading spill and says BP is responsible for the clean-up.

April 30 - An Obama aide says no drilling will be allowed in new areas, as the president had recently proposed, until the cause of the Deepwater Horizon accident is known.

-- BP Chief Executive Tony Hayward says the company takes full responsibility and will pay all legitimate claims and the cost of the clean-up.

May 2 - Obama visits the Gulf Coast. U.S. officials close areas affected by the spill to fishing for 10 days. BP starts drilling a relief well alongside the failed well, a process that may take two to three months to complete.

May 7 - An attempt to place a containment dome over the spewing well fails when the device is rendered useless by frozen hydrocarbons that clogged it.

May 9 - BP says it might try to plug the undersea leak by pumping materials such as shredded tires and golf balls into the well at high pressure, a method called a “junk shot.”

May 11/12 - Executives from BP, Transocean and Halliburton appear at congressional hearings in Washington. The executives blame each other’s companies.

May 14 - Obama slams companies involved in the spill, criticizing them for a “ridiculous spectacle” of publicly trading blame in his sternest comments yet.

May 19 - The first heavy oil from the spill hits fragile Louisiana marshlands. Part of the slick enters a powerful current that could carry it to the Florida Keys and beyond.

May 28 - Obama tours the Louisiana coast, saying “I am the president and the buck stops with me.”

-- BP’s Hayward flies over the Gulf.

May 29 - BP says the complex “top kill” maneuver, started three days earlier to plug the well, has failed.

June 1 - BP shares plunge 17 percent in London trading, wiping $23 billion off its market value, on news the latest attempt to plug the well has failed.

-- U.S. Attorney General Eric Holder says the Justice Department has launched a criminal and civil investigation into the rig explosion and the spill.

June 2 - BP tries another capping strategy but has difficulty cutting off a leaking riser pipe.

-- U.S. authorities expand fishing restrictions to cover 37 percent of U.S. federal waters in the Gulf.

June 4 - Obama, on his third trip to the region, warns BP against skimping on compensation to residents and businesses.

June 8 - Obama says he wants to know “whose ass to kick” over the spill, adding to the pressure on BP.

-- U.S. weather forecasters give their first confirmation that some of the oil leaking has lingered beneath the surface rather than rising to the top.

June 9 - U.S. Interior Secretary Ken Salazar says BP must pay the salaries of thousands of workers laid off by a moratorium on drilling, at a congressional hearing.

June 10 - In his first comments, Prime Minister David Cameron says Britain is ready to help BP deal with the spill.

-- U.S. scientists double their estimates of the amount of oil gushing from the well, saying between 20,000 and 40,000 barrels (840,000 and 1.7 million gallons/3.2 million and 6.4 million liters) flowed out before June 3.

June 11 - Supportive comments from Britain lift BP’s shares in London by 6.4 percent. But the rise does not mend the damage done -- the company is worth 70 billion pounds ($102 billion) against more than 120 billion pounds in April.

June 14 - Obama, on his fourth trip to the Gulf, says he will press BP executives at a White House meeting on June 16 to deal “justly, fairly and promptly” with damage claims.

-- Two U.S. lawmakers release a letter to Hayward saying: “It appears that BP repeatedly chose risky procedures in order to reduce costs and save time and made minimal efforts to contain the added risk.”

June 15 - Lawmakers summon top executives from Exxon Mobil, Chevron, ConocoPhillips, Royal Dutch Shell and BP.

-- Obama says in his first televised speech from the Oval Office: “But make no mistake: we will fight this spill with everything we’ve got for as long it takes. We will make BP pay for the damage their company has caused.”

June 16 - BP agrees to set up a $20 billion fund for damage claims from the spill, suspends dividend payments to shareholders and says it will pay $100 million to workers idled by the six-month moratorium on deep-sea drilling.

June 17 - Hayward faces the wrath of U.S. lawmakers as he appears before a congressional hearing. He apologizes for the spill and says everything is being done to stop it. Members of Congress accuse BP of cutting corners and ignoring warnings for the sake of profit.

June 18 - Anadarko Petroleum, part owner of the gushing well, says BP’s behavior before the blowout was “reckless” and likely represented “gross negligence or willful misconduct” that would affect obligations of the well owners under their operating agreement.

June 22 - Hayward is handing day-to-day control of the spill operation to Bob Dudley -- a reflection, says BP, of the need for the chief executive to return to other aspects of the energy giant’s business.

June 24 - A U.S. judge refuses to put on hold his decision to lift a ban on deepwater drilling imposed in response to the spill.

June 27 - Oil washes ashore in mainland Mississippi for the first time, although some had tainted its barrier islands.

June 28 - BP is forced to defend its chief executive after Russia’s deputy prime minister said he expected Hayward to resign soon.

-- BP says it is now spending $100 million a day on efforts to cap the well, clean up the spill and compensate those affected, bringing the total bill so far to $2.65 billion.