At the start of the siege, the militants gathered all Westerners into one group and tied them up, according to survivors.
The kidnappers were equipped with AK-47 rifles and put explosives-laden vests on some hostages, a U.S. State Department official said.
Some escaped by disguising themselves, according to Regis Arnoux, who runs a catering firm at the site and spoke to some of his 150 employees who were freed. He said they all were "traumatized."
Some Algerian hostages were free to walk around the site but not leave, according to Arnoux. A number of them still escaped.
As the Algerian military launched its operation Thursday, the militants moved some hostages, according to one survivor's account.
With plastic explosives strapped around their necks, these captives were blindfolded and gagged before being loaded into five Jeeps, according to the brother of former hostage Stephen McFaul.
McFaul managed to escape after the vehicle he was in -- one of several targeted by Algerian fighters -- crashed, with the explosives still around his neck, his brother told CNN from Belfast, Northern Ireland.
"I haven't seen my mother move as fast in all my life, and my mother smile as much, hugging each other," Brian McFaul said upon his family hearing his brother was safe. "... You couldn't describe the feeling."
Sadly, McFaul said the other four Jeeps were "wiped out" in an explosion, and his brother believed the hostages inside did not survive.
Nations mobilize to help citizens caught up in crisis
U.S. Defense Secretary Leon Panetta, speaking in London, said the United States was working round the clock to ensure the safe return of its citizens.
Those freed include some Americans, while other U.S. nationals are still unaccounted for, U.S. officials said.
Meanwhile, the United States is evacuating 10 to 20 people caught up in the crisis, a U.S. defense official told CNN on Friday. They will be taken to U.S. facilities in Europe, the official said, where the condition of those injured can be assessed.
Britain has sent trauma experts and consular affairs officers who can issue emergency passports to a location about 450 kilometers (280 miles) away from the besieged plant, a Foreign Office official said, so they'll be "as close" as possible to the scene.
BP said Friday that a "small number of BP employees" were unaccounted for. The same held for some workers with Statoil, though nine others with the company -- including five who escaped -- are safe. Four Norwegians and a Canadian with that oil firm are in an airport hotel in Bergen, Norway, after being transported from Algeria, Statoil spokeswoman Sissel Rinde said.
Both BP and Statoil -- two of the foreign companies with In Amenas operations -- are pulling personnel from Algeria, which is Africa's largest natural gas producer and a major supplier of natural gas to Europe.
BP said it had flown 11 of its employees and several hundred staff from other companies out of the North African country Thursday and expected another flight Friday.
Mark Cobb, a Texan who has a LinkedIn profile identifying him as general manager for a BP joint venture out of In Amenas, told CNN he was "safe," having escaped on "the first day," though he didn't elaborate.
A U.S. military C-130 plane flew 12 wounded in the ordeal out of Algeria earlier Friday, a U.S. defense official said. None of them were Americans, though efforts continue to evacuate freed Americans.
Three workers for a Japanese engineering company that was working on the site have been contacted and are safe, said Takeshi Endo, a senior manager for JGC Corp. But the company had not been able to contact 14 others, he said.