(CNN) -- Here are the latest developments Wednesday involving the BP oil spill in the Gulf of Mexico:
NEW THIS WRITE
BP Chief Executive Officer Tony Hayward says it will be 24 hours before authorities will know whether the "top kill" effort to plug the ruptured well is working. He spoke nearly four hours after the procedure got under way, and said the operation was going to plan.
PREVIOUS DEVELOPMENTS
Cleanup efforts
BP began a top kill procedure Wednesday afternoon, pumping 50,000 pounds of thick, viscous fluid twice the density of water into the pipe that is leaking in its latest attempt to stop the flow of oil.
If the procedure does not work, BP will try to fit a second, smaller containment dome over the ruptured well. A larger containment dome did not succeed in stopping the leak.
Other options include installing a second blowout preventer, a device that is supposed to plug the pipe and stop oil from gushing into the sea in the event of a problem like the one triggered when the oil rig Deepwater Horizon sank a month ago, starting the leak.
The company also could try a "junk shot," which would involve plugging the well with rubber and other debris, BP Managing Director Robert Dudley has said.
A final, permanent solution could take until August -- the estimated completion date for a relief well that is being dug on a parallel course with other efforts to stop the leak, officials say.
The National Oceanic and Atmospheric Administration has closed 22 percent of Gulf waters to fishing.
About 22,000 people are working to protect Gulf coast shorelines and wildlife.
More than 1,000 skimmers, tug boats, barges and recovery vessels are being used in cleanup.
Over 3.1 million feet of boom has been deployed to contain the spill.
BP continues to use a controversial subsea dispersant after receiving a directive from the Environmental Protection Agency to find a less toxic alternative.
More than 11.1 million gallons of oily water have been recovered.
Investigation
BP and Transocean managers had a dispute over drilling plans the day of the April 20 explosion that destroyed the oil rig Deepwater Horizon, the rig's chief mechanic recounted Wednesday to a Coast Guard-Interior Department investigative board.
The rig's installation manager and captain are scheduled to testify Thursday before the same board.
On Tuesday, congressional investigators reported that BP had three indications of trouble aboard the rig in the 51 minutes before the explosion in the form of anomalous pressure readings and surges of drilling fluid from the well. Pressure on the drill pipe "unexpectedly increased" before the blast, the committee reported, citing BP's preliminary investigation.
BP's findings also "raised concerns about the maintenance history, modification, inspection, and testing" of the blowout preventer.
Witnesses have said the explosion occurred shortly after a roar of gas erupted from the well.
Government response
President Barack Obama plans to announce Thursday "strengthened inspections" and an effort to "tighten safety regulations" for offshore drilling, an administration official told CNN Wednesday.
Obama will travel to Louisiana on Friday to observe response efforts and speak to local officials. It will be his second trip to the region since the spill began.
Plaquemines Parish, Louisiana, President Billy Nungesser called Wednesday for the resignation of U.S. Coast Guard Adm. Thad Allen, the man leading the federal government's response to the spill.
Speaking at a California solar panel factory, Obama used the oil spill to urge comprehensive energy reform.
Sen. Bill Nelson, D-Florida, is urging the government to cut "cozy" ties between the oil industry and regulators.
Interior Secretary Ken Salazar said President Obama is considering adjustments to his plan to open exploration wells for drilling in the Arctic.
The Interior Department suspended new applications for drilling permits in the wake of the Deepwater Horizon explosion.
Democratic Sens. Barbara Boxer and Benjamin Cardin called on Salazar to immediately review all offshore drilling currently under way and asked that approval of new drilling permits "include a process that ensures drilling will not pose unacceptable risks to the economy and the environment."
