April 20 (10 p.m.): Oil rig explosion. An April 21 ABCNews.com article reported, “An overnight explosion in the Gulf of Mexico rocked the Deepwater Horizon oil rig off the Louisiana coast, sending spectacular bursts of flame into the sky. The fires were still raging today.” The U.S. Coast Guard's National Oil and Hazardous Substances Response System assigns primary responsibility for cleaning up oil spills to the spiller as the responsible party.

April 21: Deputy Secretary of Interior, Coast Guard dispatched to region. An April 22 White House statement noted that following a briefing with President Obama, Department of Homeland Security Secretary Janet Napolitano, Coast Guard Commandant Adm. Thad Allen, Department of Interior Secretary Ken Salazar, EPA Deputy Administrator Bob Perciasepe, and FEMA Administrator Craig Fugate, “Deputy Secretary of the Interior David Hayes was dispatched to the region yesterday to assist with coordination and response.” The Coast Guard announced that four units were responding to the fire, with additional units en route.

• Search and rescue efforts begin for 11 missing. An initial focus of the response was the search for 11 missing crewmembers. The search was called off April 23.

• BP confirms U.S. Coast Guard was “leading the emergency response” In an April 21 press release, British Petroleum stated that it was “working closely with Transocean and the U.S. Coast Guard, which is leading the emergency response, and had been offering its help - including logistical support.”

• CNN.com: “The U.S. Coast Guard launched a major search effort.” An April 22 CNN.com article reported:

The U.S. Coast Guard launched a major search effort Wednesday for 11 people missing after a “catastrophic” explosion aboard an oil rig in the Gulf of Mexico engulfed the drilling platform in flames.

Another 17 people were injured -- three critically -- in the blast aboard the Deepwater Horizon, which occurred about 10 p.m. Tuesday. The rig was about 52 miles southeast of Venice, Louisiana, said Coast Guard Senior Chief Petty Officer Mike O'Berry. As of late afternoon Wednesday as many as six firefighting vessels were working to contain the massive fire caused by the explosion.

“It obviously was a catastrophic event,” O'Berry said.

April 23: Coast Guard “focused on mitigating the impact of the product currently in the water.” On April 23, the Coast Guard stated:

The Department of the Interior, MMS [the U.S. Minerals Management Service], and the Coast Guard continue to support the efforts of the responsible parties to secure all potential sources of pollution. Both federal agencies have technical teams in place overseeing the proposals by BP and Transocean to completely secure the well. Until that has occurred and all parties are confident the risk of additional spill is removed, a high readiness posture to respond will remain in place.

Although the oil appears to have stopped flowing from the well head, Coast Guard, BP, Transocean, and MMS remain focused on mitigating the impact of the product currently in the water and preparing for a worst-case scenario in the event the seal does not hold. Visual feed from deployed remotely operated vehicles with sonar capability is continually monitored in an effort to look for any crude oil which still has the potential to emanate from the subsurface well.

“From what we have observed yesterday and through the night, we are not seeing any signs of release of crude in the subsurface area. However we remain in a 'ready to respond' mode and are working in a collaborative effort with BP, the responsible party, to prepare for a worst-case scenario,” Landry stated early Friday morning.

April 25: Response team implements plan to contain oil spilling from source, weather delays cleanup.

• Storms delay response efforts. An April 25 Associated Press article reported, “Stormy weather delayed weekend efforts to mop up leaking oil from a damaged well after the explosion and sinking of a massive rig off Louisiana's Gulf Coast that left 11 workers missing and presumed dead.” AP further reported:

The bad weather began rolling in Friday as strong winds, clouds and rain interrupted efforts to contain the spill. Coast Guard Petty Officer John Edwards said he was uncertain when weather conditions would improve enough for cleanup to resume. So far, he said, crews have retrieved about 1,052 barrels of oily water.

• Oil recovery and cleanup were to resume after adverse weather passed. On April 25, the unified command team responding to the spill stated:

The unified command is implementing intervention efforts in an attempt to contain the source of oil emanating from the wellhead at the Deepwater Horizon incident site Sunday.

The unified command has approved a plan that utilizes submersible remote operated vehicles in an effort to activate the blowout preventer on the sea floor and to stop the flow of oil that has been estimated at leaking up to 1,000 barrels/42,000 gallons a day.

Also, BP is mobilizing the DD3, a drilling rig that is expected to arrive Monday to prepare for relief well-drilling operations.

Additionally, the oil recovery and clean-up operations are expected to resume once adverse weather has passed. These efforts are part of the federally approved oil spill contingency plan that is in place to respond to environmental incidents.

April 26: Response crews “to resume skimming operations.” On April 26, the response team stated, “Sunday, an aircrew from the U.S. Fish and Wildlife Service sighted five small whales during an over flight in the vicinity of the oil spill, which currently measures 48 miles by 39 miles at its widest points with varying levels of sheening, and is located 30 miles off the coast of Venice, La.” The command team further stated, “Following adverse weather that went through the area, response crews are anticipated to resume skimming operations today,” including 1,000 personnel, 10 offshore vessels, 7 skimming boats and more than 14,000 gallons of dispersant. At that point 48,384 gallons of oily water had been collected.

April 28: Federal officials realize spill was far more severe than BP led them to believe. An April 28 New York Times article reported, “Government officials said late Wednesday night that oil might be leaking from a well in the Gulf of Mexico at a rate five times that suggested by initial estimates." The Times further reported:

In a hastily called news conference, Rear Adm. Mary E. Landry of the Coast Guard said a scientist from the National Oceanic and Atmospheric Administration had concluded that oil is leaking at the rate of 5,000 barrels a day, not 1,000 as had been estimated. While emphasizing that the estimates are rough given that the leak is at 5,000 feet below the surface, Admiral Landry said the new estimate came from observations made in flights over the slick, studying the trajectory of the spill and other variables.

An April 30 Associated Press article reported, “For days, as an oil spill spread in the Gulf of Mexico, BP assured the government the plume was manageable, not catastrophic. Federal authorities were content to let the company handle the mess while keeping an eye on the operation.” The article continued:

But then government scientists realized the leak was five times larger than they had been led to believe, and days of lulling statistics and reassuring words gave way Thursday to an all-hands-on-deck emergency response. Now questions are sure to be raised about a self-policing system that trusted a commercial operator to take care of its own mishap even as it grew into a menace imperiling Gulf Coast nature and livelihoods from Florida to Texas.

April 29: Napolitano declares spill “of national significance”; BP insists its “plan can handle this spill.” On April 29, BP official Doug Suttles appeared on ABC's Good Morning America and stated, “At this point, I believe our plan can handle this spill, and that's what we're doing.” That day, Napolitano declared the spill “of national significance,” explaining that “we can now draw down assets from across the country, other coastal areas, by way of example; that we will have a centralized communications because the spill is now crossing different regions.”

• EPA preparing for oil to hit shore. Environmental Protection Agency Administrator Lisa Jackson commented at an April 29 press briefing: "[A]s the oil does hit the shoreline, EPA will provide support to assess the impacts on the coastal shoreline and play a key role in implementing the cleanup. As a daughter of the Gulf Coast, I know that it is our job to ensure people that we will be eyes and ears working with the states who have valuable and vital resources to monitor air, water and land quality." Jackson also stated that the EPA has deployed air-monitoring aircraft “that is gathering information on the impact of the controlled burn on air quality, both in the area of the burn, and, of course, further away.”

• AP: “Air Force sends planes to help with Gulf oil spill.” An April 30 Associated Press article reported: “Two Air Force planes have been sent to Mississippi and were awaiting orders to start dumping chemicals on the oil spill threatening the coast, as the government worked Friday to determine how large a role the military should play in the cleanup.”

• WSJ: Navy joins Obama's “robust response.” An April 30 Wall Street Journal article reported that “The U.S. Navy said it will send more than 12 additional miles of inflatable oil booms to the Gulf, as well as seven towable skimming systems and 50 contractors with experience operating the equipment.” The article continued: “The Navy is making two large facilities available to the Coast Guard personnel and BP-employed contractors who are currently taking the lead in fighting the spill. Military officials said the booms and skimmers were being sent to a Naval construction base in Gulfport, Miss. The Navy also opened its air base in Pensacola, Fla., to the effort.”