The kidnappers wielded AK-47 rifles and put explosive-laden vests on some hostages, according to a U.S. State Department official.
Algeria said the attack was in retaliation for allowing France to use Algerian airspace for an offensive against Islamist militants in neighboring Mali.
And Sahara News' report Sunday claimed Belmoktar said "40 immigrant Jihadists and supporters of Muslim countries" led the siege in retaliation for the Mali offensive.
But regional analysts believe it was too sophisticated to have been planned in just days.
On Thursday, Algerian special forces moved in because the government said the militants wanted to flee to Mali.
The Islamic extremists also planned to blow up the gas installation and rigged it with mines throughout, the U.S. official said.
Thursday's military incursion succeeded in freeing some hostages -- but not all.
Some survivors described their harrowing escapes by rigging up disguises and sneaking to safety with locals, with at least one survivor running for his life with plastic explosives strapped around his neck.
Several hostages died. And the Algerian military came under criticism from some quarters for unnecessarily endangering hostages' lives.
Undeterred, the government followed with a second push Saturday. That assault killed the remaining hostage-takers but resulted in more hostage deaths.
The army intervened "to avoid a bloody turning point of events in this extremely dangerous situation," the Algerian Interior Ministry said Saturday.
"It was clear that the terrorists were determined to escape the country with the captives and to bomb the gas installations."
On Sunday, an American lawmaker said the Algerian government turned down U.S. offers to help during the crisis.
"They decided they were going to handle it their way," said Rep. Mike Rogers, a Michigan Republican who is chairman of the U.S. House Intelligence Committee. "They did not want us or the other hostage nations involved in the decision-making."
British Defense Secretary Philip Hammond called the loss of life "appalling and unacceptable," while laying blame solely on the terrorists.
Countries mourn dead, try to track down missing
While the military part of the operation is over, the searching and mourning is not for people in countries worldwide. In addition to combing the sprawling desert site, Algerian forces are searching hospitals and medical centers around the country, as well as towns and villages near the targeted site, according to a statement Sunday from Statoil.
Colombia's president said a citizen was presumed dead.