Fourteen of BP's 18 employees at the In Amenas plant were safe after Algerian special forces stormed a natural gas complex in a final assault Saturday that ended a four-day-old hostage crisis, BP said.
The fate of the four missing BP employees appears bleak, BP CEO Bob Dudley told reporters.
"We have great fears we have likely suffered one or more fatalities," he said. For the families of the victims, "this is a terrible and agonizing ordeal," he said.
Seven hostages are believed to have been killed by terrorists during Saturday's assault, bringing the death toll to at least 55 - 23 hostages and 32 Islamist militants, according to Algerian news media.
A total of 685 Algerian and 107 foreigner workers were freed over the course of the standoff, which began on Wednesday, according to an Interior Ministry statement.
As few details emerge from the remote site in eastern Algeria, it was unclear whether anyone was rescued in the final operation.
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Military personnel are now combing the plant for mines and explosives left behind by the hostage-takers who had intended to blow up the facility, Dudley said.
"It was an unprecedented assault by a group of heavily armed murderers," he said.
There still may be survivors hiding throughout the sprawling complex, Dudley added.
Dudley said he could not provide the nationalities of those accounted for or missing but called it an "international group."
Dudley confirmed reports that one American, Frederick Buttaccio, of Katy, Texas, died during the ordeal. Another American, Mark Cobb of Corpus Christi, Texas, survived, Dudley said. Two BP employees sustained non-life-threatening injuries.
At any time, between 500 and 700 workers are on site at the In Amenas plant, mostly contractors and the workers for the local gas company, Dudley said.
Algeria's response to the crisis was typical of the country's history in confronting terrorists, favoring military action over negotiation, which caused an international outcry from countries worried about their citizens. Algerian military forces twice assaulted the two areas where the hostages were being held with minimal apparent mediation - first on Thursday and then on Saturday.
The standoff has put the spotlight on al-Qaeda-linked groups that roam such remote areas, threatening vital infrastructure and energy interests. The militants initially said their operation was intended to stop a French attack on Islamist militants in neighboring Mali - though they later said it was two months in the planning, long before the French intervention.
The militants, who came from a Mali-based group run by an Algerian, attacked the plant Wednesday morning. Armed with heavy machine guns and rocket launchers in four-wheel drive vehicles, they fell on a pair of buses taking foreign workers to the airport. The buses' military escort drove off the attackers in a blaze of gunfire that sent bullets zinging over the heads of crouching workers. A Briton and an Algerian were killed.
Afterward, the militants turned to the vast gas complex, divided between the workers' living quarters and the refinery itself, and seized hostages, the Algerian government said.
Contributing: The Associated Press
Rick Jervis and Jabeen Bhatti, USA TODAY