HOUSTON -- The companies operating the gas facility in the Sahara that was attacked last week had chosen not to deploy armed guards inside the sprawling compound, leading security analysts to question whether the assault by more than 30 Islamist militants might have been slowed if security had been tighter.
Until the siege on the remote In Amenas facility last Wednesday, dozens of North African desert camps were thought to be virtually impregnable, with steel-wire fences, long-range reconnaissance equipment and army patrols amid the sand dunes.
But when the attackers came, taking dozens of foreign workers hostage, they faced little opposition. Armed with mortars, grenade launchers and .50-caliber machine guns, the militants were an overwhelming force. But security experts said the armed guards found at most production and drilling facilities in North Africa and the Middle East might have at least slowed the terrorists, letting more workers escape.
"The attack clearly caught everybody by surprise," said Geoff D. Porter, a political risk and security consultant for oil companies in North Africa. "Had there been armed guards, there could have been a different outcome."
The Algerian government dismissed suggestions that it could have stopped the assault, which led to a major hostage crisis that left at least 37 foreigners dead.
An Algerian official, who requested anonymity because of the delicacy of the matter, said the attack was conducted in the dark by a heavily armed force that moved quickly over the border from Libya, making it hard for security forces to repel. Also, the official said, the government learned later how the militants were able to wage such a well-planned assault on the facility: one had worked there as a driver and presumably knew the layout.
Algerian law prohibits armed foreign security personnel, but it permits private Algerian armed guards. The operators of the gas facility -- a joint venture by BP, the Norwegian company Statoil and the Algerian national oil company Sonatrach -- decided to rely solely on the many Algerian gendarmes and soldiers who patrolled the In Amenas area.
Some oil companies operating in Algeria do use military guards or private Algerian security forces. "It's company-specific," said Mike Lord, chief executive of the Stirling Group, which oversaw security at the In Amenas camp. There are risks to having armed security personnel at oil or gas sites, Mr. Lord said, including explosions that might be caused by stray bullets.
Algerian security forces provided "perimeter" and "zone" security at the In Amenas base, Mr. Lord said, and the Stirling Group organized escorts of Algerian forces to accompany employees when they traveled between secured zones. But there were no armed guards within the secured zones, Mr. Lord said, under the policy set by BP and Statoil.
There was a broad fence perimeter, and it was monitored 24 hours a day. No one could come near the camp without an identification badge, and no one without official permission could travel by air or road nearby. Attacks on Algerian oil and gas sites have been rare, even during the civil war of the 1990s, officials said.
Sonatrach has a security department that employs armed guards. And according to the Norwegian newspaper Verdens Gang, Algeria gave permission in 2011, after civil war broke out across the border in Libya, for private armed guards to be used at In Amenas, one of the country's largest natural gas fields. But BP and Statoil said they did not want the legal responsibility.
"We and Statoil decided not to have armed guards on site," said Robert Wine, a BP spokesman. "Given the large military presence in the area, we took the view that armed guards were not required on the site."
A Statoil spokesman, Bard Glad Pedersen, said the company would review its security procedures. "We will go through all elements of this terrible event, including questions connected to security," he said in an e-mail.
That review should be conducted by an independent panel, said Stein Bredal, a former member of the Statoil board, who contends that the company underestimated the risks in Algeria. "To be sure that the truth really comes forward, it's much better that people can speak frankly, and they have a duty to report whatever they've been through," he said in an interview.
Bernard Duncan Lyng, who was head of corporate security and contingency units at Statoil from 1984 to 1992 and worked with the company until 2000, said that when Statoil set up a facility in Nigeria in the early 1990s, it hired armed guards.
"The first thing we did was to plan where would the best place be to have our site, safety- and security-wise, and establish a system of armed guards, and we did that," Mr. Lyng said.
But while rare, attacks on oil company personnel in Algeria have occurred. In 1994, during the civil war between Algiers and Islamist militants, two Schlumberger engineers were executed at a Sonatrach site by militants.
Most big oil and gas companies have armed guards in their facilities, said Pierre Montoro, managing director of Erys Group, a French company that provides security to multinational companies in Algeria and around the region. "People should be armed on these sites to contain the attack, for a few minutes, anyway, to allow the authorities to intervene."
Executives in the oil and gas industry said that their companies could handle the extra security concerns, but that new defense measures would be costly.
"To recreate confidence, it will take a lot of hand-holding with insurance companies, service companies and investment bankers," said Badr Jafar, president of Crescent Petroleum, a regional oil company based in the United Arab Emirates.
Others were more dire in their predictions of higher expenses, and ultimately higher prices at the pump. "This is going to drive up costs probably 20 percent," said Dragan Vuckovic, president of Mediterranean International, an oil service company that operates in Libya and Iraq.
Clifford Krauss reported from Houston and Nicholas Kulish from Bergen, Norway. Stanley Reed contributed reporting from London, Adam Nossiter from Algiers and Scott Sayare from Paris.
Correction: January 24, 2013, Thursday
This article has been revised to reflect the following correction: An earlier version of this article misstated the number of foreign hostages who were killed last week at a gas facility in the Algerian Sahara. At least 37 foreign hostages were killed, not 27.
This article originally appeared in The New York Times.