Western intelligence agencies and analysts for years have been warning that al Qaeda and other terrorist groups are increasingly relying on a deadly weapon in their tool kits: female suicide bombers.
Those concerns were underscored Sunday when a suicide bomber, suspected to be a woman, detonated explosives at a busy railway station in the southern Russian city of Volgograd, killing at least 15 people and wounding dozens.
No group claimed responsibility for the attack, but suspicions have fallen on Islamist rebels from Chechnya.
"The use of female suicide bombers is a classic Chechen tactic, which is increasingly popular with other jihadist groups," said Bruce Riedel, a former CIA officer who heads the Intelligence Project at the Brookings Institution in Washington.
The attack took place as Russia prepares to host the Winter Olympics in the Black Sea resort of Sochi in February and was the second terrorist attack in southern Russia in three days. A bomb in a parked car killed three people Friday in Pyatigorsk.
In June, Doku Umarov, the leader of the Islamist insurgency in the North Caucasus, lifted a moratorium on terrorist attacks inside Russia and in a videotaped message instructed rebels to use "maximum force" to disrupt the Winter Games.
"They are planning to hold the games on the bones of many, many Muslims buried on our land by the Black Sea. It is incumbent on us as Muslims not to permit that, resorting to any methods Allah allows us," said Umarov, who is Chechen.
Sochi is about 250 miles from the republics of Chechnya and Dagestan, where the Islamist rebels are based. Volgograd, the city formerly known as Stalingrad and best known for a decisive Soviet World War II victory over Nazi Germany, is more than 500 miles from both restive Muslim republics.
"The Sochi Olympics are an extremely attractive target for al Qaeda and like-minded groups who want to bring attention to the Chechen cause," said Mr. Riedel. "We should anticipate an effort to surge attacks on Russian targets at home and abroad in the next weeks."
Female suicide bombers often exact higher tolls than their male counterparts simply because their movements are less scrutinized.
"To the extent security forces are looking mostly for male terrorists — which most terrorists have been — females are better able to evade scrutiny and slip into a crowd," said Paul R. Pillar, nonresident senior fellow at Georgetown University's Center for Security Studies.
"In some parts of the world, typical female clothing may be better able to conceal a bomb than is male clothing," he said.
The Obama administration condemned "in the strongest terms" the attack in Volgograd, State Department spokeswoman Jen Psaki said Sunday.
"We send our sincere condolences to the families of the victims and stand in solidarity with the Russian people against terrorism of any kind," she said.
Russian officials said the suicide bomber in Volgograd was a woman. One report identified her as Oksana Aslanova, a widow of a Dagestani militant.
Late Sunday, the Interfax news agency quoted an unidentified law enforcement officer as saying surveillance camera footage showed that the bomber was a man. There was no official confirmation on the bomber's identity.
Chechen women have carried out almost half of the suicide attacks in Russia since 2000. Their attacks turned so frequent that they were dubbed "black widows" after a pattern showed that many of them were acting to avenge the deaths of their husbands, sons and brothers.
In October, Volgograd was attacked by a female suicide bomber who killed six people and injured about 30 on a city bus.
The first Chechen suicide bombers were two women — Khava Barayeva and Luiza Magomadova — who on June 7, 2000, drove a truck packed with explosives into a Russian special forces building in the village of Alkhan-Yurt in Chechnya. The rebels claimed more than two dozen people died. Russian authorities put the death toll at two.
In October 2002, 19 women were among the 40 Chechen terrorists who held nearly 1,000 people hostage at Moscow's Dubrovka Theater. All 40 terrorists and 130 hostages died after Russian special forces pumped sleeping gas into the theater and stormed it. In March 2010, two female suicide bombers attacked the Moscow subway, killing 40 people and wounding more than 120.
In September 2004, terrorists led by Khaula Nazirov, a 45-year-old "black widow" from the Chechen capital Grozny, attacked a school in Beslan, a town in North Ossetia. More than 300 people, most of them children, died in the three-day siege.
Western intelligence agencies and analysts are concerned that al Qaeda, which has ties to the Chechen rebels, is training and deploying female suicide bombers.
In 2010, law enforcement agents were on the lookout for female suicide bombers linked to al Qaeda in the Arabian Peninsula, who were thought to be planning to enter the U.S.
Former White House counterterrorism adviser Richard A. Clarke said at the time that al Qaeda had "trained women."
"There are others who are still out there who have been trained and who are clean skins — that means people who we do not have a record of, people who may not look like al Qaeda terrorists, who may not be Arabs, and may not be men," Mr. Clarke told ABC News.
Russian officials told reporters that the bomber detonated her device — containing 22 pounds of TNT and packed with injury-causing shrapnel — outside the metal detectors that guard the station entrance.
"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," said Vladimir Markin, a spokesman for the nation's top investigative agency.
Sochi will have extremely tight security for the Winter Olympics. In addition to the admission ticket, each spectator must obtain a separate pass that requires providing Russian authorities with contacts and passport information. Cars from outside a security zone covering thousands of square miles around Sochi will not be permitted into that zone for a month before the games and a month afterward.
"We have no doubt that the Russian authorities will be up to the task" of Olympic security, the International Olympic Committee said in a statement expressing concern about the Sunday bombing.
The first-known female suicide bomber was likely Sana'a Youcef Mehaidli, a 16-year-old member of the secular Syrian Social Nationalist Party. On April 9, 1985, she rammed a vehicle packed with explosives into an Israeli Defense Forces convoy in southern Lebanon, killing two soldiers.
Female suicide bombers have been used by militant groups, including the Palestinian Hamas and Islamic Jihad against Israelis, al Qaeda in Iraq, the Taliban in Afghanistan, and the Kurdistan Workers' Party in Turkey, but the practice is not exclusive to Islamists.
"It is not a group-specific tactic," Mr. Pillar said.
"To the extent that any one group became identified more than others with this particular tactic, it was probably the Tamil Tigers when it still existed," said Mr. Pillar, a CIA veteran and former national intelligence officer for the Near East and South Asia.
The Tamil Tigers — a secular, Marxist-Leninist separatist organization of Hindu Tamils — relied immensely on suicide bombers in their nearly 26-year war against the military in Sri Lanka that ended with the rebels' defeat in May 2009.
A female suicide bomber was responsible for one of the Tamil Tigers' most high-profile attacks when she assassinated former Indian Prime Minister Rajiv Gandhi on May 21, 1991.
In his book "Dying to Win: The Strategic Logic of Suicide Terrorism," Robert A. Pape contends that suicide terrorism is largely a response to foreign occupation.
"Few suicide attackers are social misfits, criminally insane, or professional losers," writes Mr. Pape, a professor of political science at the University of Chicago.
"Most fit a nearly opposite profile: typically they are psychologically normal, have better than average economic prospects for their communities, and are deeply integrated into social networks and emotionally attached to their national communities. They see themselves as sacrificing their lives for the nation's good," he adds.
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