Timeline: Gulf of Mexico oil spill

LA/WASHINGTON (Reuters) - Millions of gallons/liters of oil have poured into the Gulf of Mexico since an April 20 blast on the Deepwater Horizon rig triggered a huge spill, soiling 100 miles of coastline, threatening some of the richest U.S. fisheries and endangering a fragile marine ecosystem.

Below is a timeline of the spill and its impact.

April 20, 2010 - Explosion and fire on Transocean Ltd's drilling rig Deepwater Horizon licensed to BP; 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-long oil slick forms.

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

April 28 - The Coast Guard says the flow of oil is 5,000 barrels per day (210,000 gallons/795,000 liters) -- five times greater than first estimated.

April 29 - U.S. President Barack Obama pledges "every single available resource," including the U.S. military, to contain the spreading spill and that BP is responsible for the cleanup. Louisiana declares state of emergency.

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 cleanup.

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 will take two to three months to complete.

May 6 - Oil washes ashore on the Chandeleur Islands off the Louisiana coast, uninhabited barrier islands that are part of the Breton National Wildlife Refuge.

May 7 - An attempt to place a containment dome over the spewing well fails with the device rendered useless by frozen hydrocarbons that clogged it. A fishing ban for federal waters is expanded and extended.

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 over the accident in his sternest comments yet.

May 16 - BP inserts a tube into the leaking riser pile of the well and captures some oil and gas.

May 18 - The U.S. nearly doubles a no-fishing zone, extending it to 19 percent of U.S. waters in the Gulf.

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 26 - A "top kill" maneuver starts, involving pumping heavy fluids and other material into the well shaft to try to stifle the flow.

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

-- BP CEO Tony Hayward flies over the Gulf. BP says the disaster has cost the company $930 million.

May 29 - BP says the complex "top kill" maneuver to plug the well has failed, crushing hopes for a quick end to the largest oil spill in U.S. history on its 40th day.

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 3 - After previous sharp declines, BP shares rise more than 3 percent on market hopes of progress in controlling the leak. The 6-week-old crisis has wiped a third off BP's market value since it began.

June 4 - BP's containment cap is said to be collecting about 1,000 barrels per day. The government estimates 19,000 barrels a day could be gushing.

-- Tar balls wash ashore in northwest Florida, the first apparent impact there from the spill.

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

June 6 - BP say its latest effort had captured 10,500 barrels of oil (439,950 gallons/1.67 million liters) in 24 hours and a second containment system should soon enable it to control the vast majority of oil.

June 7 - BP, which says it has now spent $1.25 billion on the spill, sees shares gain on news of the progress in containing the leak but still faces tough questions from investors and U.S. lawmakers.

(Compiled by David Cutler, London Editorial Reference Unit; Editing by Pascal Fletcher and Bill Trott)