In the wake of the terrorist bombings of two Russian airliners and a suicide bombing outside a Moscow subway station, U.S. officials are insisting on tighter security for all flights from Russia to the United States. The three terrorist attacks are being blamed on Chechen separatist militants. The suspected perpetrators were women and that presents new challenges in the counter-terrorism campaign.

Chechen militants fighting for an independent republic have not hesitated to include women in their operations. Women took part in the hostage operation two years ago in a Moscow theater and in other operations leading up to the recent spate of suicide bombings.

Analyst Olga Oliker of the Rand Corporation is not surprised. She says it is another tactic to grab attention and terrify the general public.

"In some ways it's more terrifying for the public they are trying to frighten. We're used to profiling potential terrorists as a certain group of people and that group of people is generally male. So to also have to look at women and young girls creates a much bigger challenge for authorities," she says.

The motivation for Chechen female bombers is not clear. Ms. Oliker says some, like their male counterparts, often are caught up in the fervor of a cause. Some are forced into suicide operations. Others, who are known as "black widows", are seeking revenge for the death of a husband, father, brother or son.

Stanley Bedlington is a former CIA senior analyst on counterterrorism. He says women also are harder to detect because of ingrained social attitudes toward women.

"They are chosen because modesty will prevent some people from searching them so they can slip into the target area. And therefore in some instances, women terrorists with explosive belts around them can get through the security guards," he says.

Some analysts suggest the Chechen terrorists also are emulating Palestinian militants who have deployed female bombers to great effect. Amatzia Baram of Haifa University agrees that the number of female suicide bombers in the Arab-Israeli conflict have increased.

"It's like something catching, a disease. If many men are killing themselves this way, eventually some women will be drawn into it too and become legitamate," he says.

Mr. Baram, a respected analyst of political Islam, says in the past more religiously-oriented militant organizations like Hamas have been more reluctant to use female suicide bombers than more secular groups like Fatah fighters. But, he adds, that is changing.

"Fatah never had any inhibitions. They used women and usually they found them in a very deep personal distress or crisis and they used them. Hamas didn't do it. I think the first time when they sent a young woman, a mother of three to explode herself at the border crossing between Gaza and Israel was a few months ago," he says.

Counter-terrorism expert Bedlington stresses that female terrorists are not a recent development and religion is not a factor.

"It's not new at all. If you go back to the heyday of the secular terrorists who were Marxists, that is the Red Brigades and the Red Army faction in Germany who were all Marxists and not at all driven by religious terrorism, it was more or less the heyday of women terrorists," he says.

In the Middle East, it was a woman, Layla Khalid, who carried out one of the first airline hijackings in 1969 to draw attention to the Palestinian cause. Amatzia Baram also remembers one of the first suicide bombings in the 1980s was carried out by a woman in Lebanon.

"The first case of suicide bombing that I know of in Israel or around Israel happened in Lebanon when the Israeli forces were still there. In 1984 I think when a woman member of a very secular Christian Pan-Syrian party exploded herself and killed a few of the Israeli soldiers," he says.

Russia expert Olga Oliker says the challenge for security officials is how to deal with the female factor.

"People aren't used to looking at women as potential bombers. And you need to train people to pay attention to girls as well as boys and to see women as potential attackers," she says.

Ms. Oliker says it will also require a dramatic psychological shift in the mindset of security forces to view women now as much as men as a potential adversary.