BP oil spill: Gulf flight marks year since disaster
Families of the 11 men who died in the Deepwater Horizon oil rig explosion have flown over the Gulf of Mexico, where the disaster happened a year ago.
On land, candlelight vigils were held in states affected by the spill, marking the first anniversary.
US President Barack Obama paid tribute to those who died and the thousands involved in the operation to clean up after the worst US oil spill.
While "progress" had been made, Mr Obama said, "the job isn't done".
"We continue to hold BP and other responsible parties fully accountable for the damage they've done and the painful losses that they've caused," Mr Obama said in a statement.
"Today, we remember the 11 lives lost as a result of this tragic event and thank the thousands of responders who worked to mitigate this disaster."
The accident, jointly blamed on BP, Halliburton and Transocean, affected the lives of millions of people in the states of Louisiana, Mississippi, Alabama and Florida.
Courtney Kemp, whose husband Roy Wyatt Kemp was killed on the Deepwater Horizon rig, wrote on her Facebook page on Tuesday that she could not "believe tomorrow has been one year because it seems like everything just happened".
She added: "I have learned a lot of things through all of this but the most important is to live each day as if it were your last... what matters is if you truly live."
Transocean, which owned the rig, invited up to three members of each family to participate in the flyover in the Gulf.
The families were expected to circle the site of the disaster several times in a helicopter.
Eleven stars representing the victims lost in the explosion have been imprinted on the well's final cap.
A fund set up by the oil company BP to compensate those affected by the environmental damage has paid out almost $4bn (£2.4bn), but some of those affected say arrangements have been slow and unfair.
BP has sued the maker of a blowout preventer that it alleges supplied a faulty part, leading to the spill.
The oil firm is seeking damages to help fund the compensation and clean up costs.
The BBC's Andy Gallacher, who has just been in the region, says that 12 months later the healing process has been slow in the Gulf.
Tourists are beginning to return to the area, but for many still seeking compensation there is continued frustration and a loss of faith in both BP and the Obama administration, our correspondent adds.
Many questions also remain about the environmental damage. Most scientists agree the disaster was not as severe as first feared, but the long term effects of such a huge catastrophe will take years to unfold.
The last of the fishing waters that were closed last year have finally been re-opened but for the people of the Gulf states, the scars are far from healed, our correspondent says.
"I don't see any daylight at the end of this tunnel. I don't see any hope at all. We thought we'd see hope after a year, but there's nothing," Audrey Neal, who makes her living from crabbing in the Gulf, told the Associated Press news agency.
Meanwhile, some residents and politicians in Louisiana have criticised Mr Obama's response to the disaster - which included a temporary moratorium on new deepwater drilling until new safety rules could be implemented - claiming it has had a greater negative impact on the state than the spill did.
"When we have other tragedies - when we have airlines that crash, we don't shut down the airline industry," Louisiana Governor Bobby Jindal said on Wednesday.
Mr Jindal has urged federal regulators to speed up the permit process and allow deepwater drilling rigs to go back to work.
Activists at a candle-lit sunrise vigil in New Orleans voiced a different opinion on Wednesday, saying the disaster should be a viewed as a catalyst to push the US towards greater use of alternative energy sources.
"The only way to protect our communities, our waters and our air from another disaster is to break our addiction to oil and embrace a cleaner, safer energy future for America," said Robin Mann, president of the Sierra Club environmental group.
Overnight on 20 April 2010, the Transocean Deepwater Horizon burst into flames while drilling a well for BP.
Eleven workers on or near the drilling rig were killed, their bodies never recovered. The rest of the crew evacuated, and two days later the rig toppled into the sea.
In the weeks that followed, more than 200 million gallons (780 million litres) of oil flowed in the Gulf of Mexico from the well - making it the worst US oil spill in recent history.