The slain American was identified as Frederick Buttaccio of Texas.
Relieved to have their freedom restored, hostages, many of them Algerian, were seen hugging loved ones on Algeria's state news broadcast. The agency says 650 hostages have been freed, but about 30 foreigners are still unaccounted for. The U.S. state department says some Americans are still being held hostage.
"We are working around the clock to ensure the safe return of our citizens," said U.S. Defense Secretary Leon Panetta.
Some freed hostages are on their way back home to the UK, while others needing medical attention have been flown out of the region on U.S. military planes to a medical facility in Europe.
The siege began Wednesday when an estimated 20 gunmen attacked a bus carrying a group of international workers. The bus was escorted by two cars of security teams, but during the strike, at least one worker was killed.
The terrorists moved next to the compound, jointly operated by BP and a Norwegian company, where they remain hunkered down with hostages from the U.S., Algeria, Norway, Japan, France and other countries.
Algerian helicopters launched a rescue mission, but it was not entirely successful. Algerian state news says 12 hostages were killed during the raid.
Prior to the raid, five Americans managed to escape their captors. U.S. officials say the situation is still extremely fluid.
"We will not rest until we do as much as we can alone and in concert with our partners to restore security to this vital region," said Secretary of State Hillary Clinton.
A Mauritanian news site that frequently receives messages from al Qaeda-linked militants said the hostage-takers in Algeria had offered to trade two captive Americans for two jailed terror figures in the United States. One of the two, Omar Abdel Rahman, masterminded the 1993 World Trade Center bombing.
According to a news agency in the region, the attack was in the works for two months. Members of the Mali-based al Qaeda offshoot known as the Masked Brigade were armed with rocket launchers and machine guns. Algerian officials say the militants entered through southern Libya.
ABC News and the Associated Press contributed to this report.