A summary of events on Thursday, May 27, Day 36 of the Gulf of Mexico oil spill that began with the April 20 explosion and fire on the drilling rig Deepwater Horizon, owned by Transocean Ltd. and leased by BP PLC, which is in charge of cleanup and containment. The blast killed 11 workers. Since then, oil has been pouring into the Gulf from a blown-out undersea well at a rate of at least 210,000 gallons per day.


BP's attempt to choke off the gusher at the bottom of the Gulf of Mexico appeared to be making some progress, officials said Thursday. BP started shooting heavy drilling mud into the blown-out well 5,000 feet underwater on Wednesday afternoon. BP said it should know by late Friday if the technique worked.


Dire new government estimates showed the disaster has easily eclipsed the Exxon Valdez as the biggest oil spill in U.S. history. A team of government scientists said the oil has been flowing at a rate 2½ to five times higher than BP and the Coast Guard had estimated. Two teams of scientists calculated the well has been spewing between 504,000 and more than a million gallons a day — or 18 million to 39 million gallons so far. That larger figure would be nearly four times the size of the Exxon Valdez disaster, in which a tanker ran aground in Alaska in 1989, spilling nearly 11 million gallons. The spill is not the biggest ever in the Gulf. In 1979, a drilling rig in Mexican waters — the Ixtoc I — blew up, releasing 140 million gallons of oil.


President Barack Obama announced major new restrictions on drilling projects. He also extended a freeze on new deepwater oil drilling and canceled or delayed proposed lease sales in the waters off Alaska and Virginia and along the Gulf Coast.


Obama says there was too much complacency in the government before the Gulf Coast oil spill and not enough focus on what could happen in a worst-case scenario. He says failing to anticipate worst-case scenarios has turned out to be a problem.


Interior Secretary Ken Salazar says the government will require independent certification of emergency cutoff valves on offshore oil wells like those that failed in the massive oil spill in the Gulf of Mexico. Salazar said those rules will be part of an effort to tighten oversight of offshore drilling.


Obama said he would end the "scandalously close relationship" between regulators and the oil companies they oversee. Elizabeth Birnbaum, head of the Minerals Management Service since July, resigned under pressure. Her agency has long been criticized for lax oversight of drilling and cozy ties with industry.


Federal officials say cleaning up the massive oil spill in the Gulf of Mexico has already cost the government $87 million, making it the third-most expensive cleanup effort in the nation's history. Coast Guard Rear Adm. Mary Landry has distributed that money to state and federal agencies directly involved in the cleanup. A senior financial analyst at the National Pollution Funds Center says another $38 million in emergency money has been assigned to the Deepwater Horizon spill, but has yet to be spent.


Marine scientists said they have spotted a huge new plume of what they believe to be oil deep beneath the Gulf, stretching 22 miles from the leaking wellhead northeast toward Mobile Bay, Ala. They fear it could have resulted from using chemicals a mile below the surface to break up the oil. The discovery by researchers on the University of South Florida College of Marine Science's Weatherbird II vessel is the second significant undersea plume recorded since the Deepwater Horizon exploded on April 20.


The head of the Environmental Protection Agency says that BP's use of dispersants to fight the Gulf oil spill has been significantly reduced. EPA Administrator Lisa Jackson told a House subcommittee on Thursday that BP used less than 12,000 gallons of dispersants on Wednesday, down from 70,000 gallons four days ago.


Seven workers helping to clean up the Gulf oil spill were hospitalized after they reported dizziness, headaches and nausea. West Jefferson Medical Center spokeswoman Taslin Alfonzo said Thursday that the emergency room doctor said her symptoms were typical of chemical exposure. One was released Thursday, and doctors were considering whether to release five more.


U.S. Coast Guard Admiral Thad Allen on Thursday approved parts of Louisiana's $350 million plan to ring its coastline with a wall of sand meant to keep out the Gulf of Mexico oil spill. Allen said the state could move forward on a network of sand berms along the Chandeleur Islands and a string of barrier islands west of the Mississippi River. They make up about half of an 86-mile network considered a last-ditch attempt to keep oil out of the state's fragile marshes. Louisiana Gov. Bobby Jindal said BP PLC will pay for the first section — and for five more if the U.S. Army Corps of Engineers deems the effort successful.


An environmental group asked a federal appellate court to halt 49 offshore drilling plans in the Gulf of Mexico that were approved without full environmental review. The Center for Biological Diversity's lawsuit challenges exploration and drilling plans approved within the past 60 days that were exempted from full environmental analyses required under the National Environmental Policy Act. BP received a similar exemption before a blowout on its Deepwater Horizon platform last month killed 11 workers and triggered a massive spill.


A ship that worked offshore in the Gulf oil spill entered Mobile Bay with dark gunk on its hull, angering residents who believe any vessels should be cleaned before coming into the bay. The Alabama State Port Authority said the 454-foot pipe-laying ship had an initial scrubbing at sea before entering the bay. It denied an allegation that the work violated pledges that ships smeared with oil wouldn't be allowed into the bay.


Two workers injured when an oil rig exploded in the Gulf of Mexico tell Congress that the companies in charge of the doomed drilling operation cut corners and neglected maintenance in a race for higher profits. Laborer Stephen Stone told the House Judiciary Committee Thursday that the companies gambled with workers' lives. But Jimmy Harrell, the Deepwater Horizon's offshore installation manager, told a panel of Coast Guard and Minerals Management Service officials at a separate hearing near New Orleans that he didn't feel pressured at all.