The government said after the assault that at least 32 extremists and 23 hostages were killed. Then, on Sunday, Algerian bomb squads sent in to blow up or defuse the explosives found 25 bodies, said the security official, who spoke on condition of anonymity because of the sensitivity of the situation.
"These bodies are difficult to identify. They could be the bodies of foreign hostages or Algerians or terrorists," the official said.
In addition, a wounded Romanian who had been evacuated died, raising the overall death toll to at least 81.
"Now, of course, people will ask questions about the Algerian response to these events, but I would just say that the responsibility for these deaths lies squarely with the terrorists who launched a vicious and cowardly attack," British Prime Minister David Cameron said. Three Britons were killed and another three were feared dead.
The dead were also known to include American, Filipino and French workers. Algerian authorities said 685 of their citizens, the backbone of the workforce, escaped without saying how many may have died. More than two dozen foreigners were unaccounted for.
It was unclear whether anyone was rescued in the final assault on the complex, which is run by the Algerian state oil company along with BP and Norway's Statoil.
Authorities said the bloody takeover was carried out Wednesday by 32 men from six countries, under the command from afar of the one-eyed Algerian bandit Moktar Belmoktar, founder of the Masked Brigade, based in neighboring Mali. The attacking force called itself "Those Who Sign in Blood."
The militants initially said the operation was payback for French military intervention in neighboring Mali, where al-Qaida-linked rebels are on the march, but later they said it was two months in the planning, long before France sent in troops.
Armed with heavy machine guns, rocket launchers, missiles and grenades, the militants singled out foreign workers at the plant, killing some of them on the spot and attaching explosive belts to others.
Algeria's tough and uncompromising response to the crisis was typical of its take-no-prisoners approach in confronting terrorists, favoring military action over negotiation. Algerian military forces, backed by attack helicopters, launched two assaults on the plant, the first one on Thursday.
The militants had "decided to succeed in the operation as planned, to blow up the gas complex and kill all the hostages," Algerian Communications Minister Mohamed Said said in a state radio interview.
The Masked Brigade claimed responsibility for the attack in the name of al-Qaida, according to a video obtained by a Mauritania-based website that sometimes carries messages of jihadists.
An audio recording of Algerian security forces speaking with the head of the kidnappers, Abdel Rahman al-Nigiri, on the second day of the drama indicated the hostage-takers were trying to organize a prisoner swap.
"You see our demands are so easy, so easy if you want to negotiate with us," al-Nigiri said in the recording broadcast by Algerian television. "We want the prisoners you have, the comrades who were arrested and imprisoned 15 years ago. We want 100 of them."
In another phone call, al-Nigiri said that half the militants had been killed by the Algerian army on Thursday and that he was ready to blow up the remaining hostages if security forces attacked again.
SITE Intelligence Group, which monitors videos from radicals, posted one showing al-Nigiri with what appeared to be an explosive belt around his waist.
The Algerians' use of forced raised an international outcry from some countries worried about their citizens.
But French Foreign Minister Laurent Fabius said Sunday on French television: "The terrorists ... they're the ones to blame."
David Plouffe, a senior adviser to President Barack Obama, said that al-Qaida and al-Qaida-affiliated groups remain a threat in North Africa and other parts of the world, and that the U.S. is determined to help other countries destroy those networks.
Speaking on "Fox News Sunday," Plouffe said the tragedy in Algeria shows once again "that all across the globe countries are threatened by terrorists who will use civilians to try and advance their twisted and sick agenda."