The growing use by terrorist groups of women — some disguised as expectant moms — to deliver deadly homicide bombs has prompted the Department of Homeland Security and FBI to issue a rare warning that such attacks could take place on American soil.

The joint security assessment cited recent female homicide bomber attacks in Baghdad — in which two women who appeared to have Down syndrome delivered a deadly explosion that killed 99 — as well as in Sri Lanka, Chechnya, India, Pakistan and the Palestinian territories as reason for the warning.

"Female suicide bombers may have an advantage over their male counterparts in accessing targets," the analysis cautioned. "The means to conduct a suicide attack vary widely, but a key element in maximizing the lethality of a suicide bombing is the bomber's ability to get close to the target."

The assessment also strongly warned that potential female homicide bombers could use "prosthetic devices that mimic the look of a pregnant woman."

"Regardless of delivery by a man or a woman," the warning reads, "improvised explosive devices may contain fragmentation or shrapnel, such as nails, bolts, glass fragments, marbles, ball bearings, or other small metal pieces."

The assessment, which emphasizes that DHS and FBI have no solid evidence indicating imminent homicide bombing attacks on U.S. soil, also reminds law enforcement officials that "facilities such as public places" are the most vulnerable to homicide bombing attacks and that "the terrorist's latitude in determining and adjusting the target and timing of an attack up to the point of detonation" make it difficult to prepare countermeasures.

"These factors indicate the importance of ... alertness by security professionals to potential threats from the full range of gender and age groups," the bulletin reads.

While homicide bombings within the U.S. have yet to occur, law enforcement agencies have increased their vigilance. In one near-attack in 1997, a Palestinian immigrant came within hours of detonating himself in a crowded Brooklyn, N.Y., subway station used by many Orthodox Jews. His roommate discovered the plot and tipped off police, who were able to prevent the bombing.

Last June, Al Qaeda and Taliban-trained homicide bombers reportedly were dispatched to the U.S., Great Britain, Canada and Germany following a videotaped graduation ceremony that included congratulations from Taliban commander Monsoor Dadullah.

Pakistani security forces reported Monday that they had captured Dadullah in a gunfight at the Afghanistan-Pakistan border.