Factbox: Chronology of Gulf of Mexico oil spill

(Reuters) - A drilling rig explosion on April 20 left 11 workers missing and presumed dead, and the rig's subsequent collapse unleashed a major oil spill that threatens the ecosystems and economy of the U.S. Gulf of Mexico.

Also threatened is the heart of U.S. energy production, both on and offshore, as a giant, unprecedented underwater leak spreads oil across the northern Gulf of Mexico between the mouth of the Mississippi River and Florida.

President Barack Obama's plans to widen offshore drilling have been suspended, and energy giant BP Plc faces another blow to its reputation and a multibillion dollar bill for cleaning up the mess and paying damages.

Below is a chronology 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 missing, 17 injured. 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. A blowout preventer, intended to prevent release of crude oil, failed to activate.

* April 22 - The Deepwater Horizon rig, valued at more than $560 million, sink and a five mile long oil slick is seen.

* April 23 - The U.S. Coast Guard suspends search for missing workers. All were thought to have been near the site of the blast and are presumed dead.

* April 25 - The Coast Guard says remote underwater cameras detect the well is leaking 1,000 barrels of crude oil per day. The agency calls the leak a "very serious spill" that threatens ecosystems along the Gulf Coast. It approves a plan to have remote underwater vehicles activate a blowout preventer and stop leak. Efforts to activate the blowout preventer fail.

* April 26- BP's shares fall 2 percent on fears that the cost of cleanup and legal claims will deal the London-based energy giant a heavy financial blow.

* April 27 - U.S. departments of Interior and Homeland Security announce plans for a joint investigation of the explosion and fire. The Coast Guard says the leaking crude may be set ablaze to slow the spread of oil in the Gulf.

* April 28 - The Coast Guard says the flow of oil is 5,000 barrels per day (bpd) (210,000 gallons/795,000 liters) -- five times greater than first estimated. Controlled burns begin on giant oil slick, but shifting winds are expected to push crude ashore.

* April 29 - Obama pledges "every single available resource," including the U.S. military, to contain the spreading spill, which Homeland Security Secretary Janet Napolitano says is of "national significance." Obama also says BP is responsible for the cleanup. Louisiana declares state of emergency due to the threat to the state's natural resources.

* 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 Chairman Tony Hayward says the company takes full responsibility for the spill and would pay all legitimate claims and the cost of the cleanup. The Interior Department orders safety inspections of all 30 deepwater drilling rigs and 47 deepwater production platforms.

* May 1 - Coast Guard says leak will affect the Gulf shore.

* May 2 - Obama visits the Gulf Coast to see cleanup efforts first hand. U.S. officials close areas affected by the spill to fishing for an initial period of 10 days.

* May 5 - A barge begins towing a 98-ton containment chamber to the site of the leak. BP says one of the three leaks has been shut off by capping a valve, but that would not cut the amount of oil gushing out. Officials plan a controlled burn to remove oil from the open water, the first since April 28.

* 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 and important nesting and breeding areas for many bird species.

* May 7 - BP engineers use undersea robots to move the containment chamber over the larger of the two remaining leaks on the seabed. The firm abandons efforts to close valves on a failed blowout preventer with underwater robots. A fishing ban for federal waters off the Gulf is modified, expanded and extended to May 17.

* May 8 - BP's containment dome hits a problem when a buildup of crystallized gas forces engineers to postpone efforts to place the chamber over the oil leak and siphon oil to the surface. "Tar balls" suspected to come from the leak wash up along a half-mile stretch of Dauphin Island, Alabama.

(Reporting by Erwin Seba and Ros Krasny; editing by Chris Wilson)