U.S. law enforcers have revealed an astonishing new phenomenon that is complicating their counterterrorism efforts—the growing number of women, even teenagers, who are being drawn to the online propaganda of the Islamic State (ISIS).

Although it is unusual for a woman, at least in the U.S., to be involved in mass shooting, the fact that a 29-year-old Muslim mother, together with her husband, perpetrated the San Bernardino massacre poses a new and more worrisome headache for law enforcers, sources told the Associated Press.

They said the increasing number of women supporting the ISIS' jihad call is making it difficult for law enforcers to tag terrorist suspects.

Even much harder to detect is a husband and wife terrorist tandem, like that of Syed Rizwan Farook and his wife Tashfeen Malik, the shooters in Wednesday's San Bernardino massacre, investigators said. This is because the two didn't have to use a telephone to plan their attacks since they lived in the same house.

FBI Director James Comey said law enforcers have a huge problem on their hands since they have "a wide spectrum of folks" to look at. "It isn't a particular demographic or geography. It's about people seeking meaning in their lives in a misguided way," Comey said.

Although male ISIS supporters in the U.S. still outnumber female Islamist radicals, the latter's number is growing, authorities said.

Before the California attacks, the Anti-Defamation League had identified 15 women linked to Islamic extremist activity in 2014 and 2015 — a number higher than that of the previous decade.

A recent George Washington University report on ISIS revealed that one-third of the nearly 300 Twitter accounts of U.S.-based ISIS supporters monitored during a six-month period appeared to belong to women.

Many of ISIS' female U.S. recruits had tried to join the jihadist group in its de-facto capital in Raqqa, Syria, officials said. They included three teenage girls from Colorado who were intercepted in Germany last year. A 19-year-old Mississippi woman also tried to join her ISIS fighter-boyfriend but was stopped by FBI agents at a regional airport.

Other female ISIS recruits were accused of planning violence. In New York, two women were charged in April with plotting to build a bomb for an attack, AP reported.