Two Americans are sitting in jail cells today, accused of aiding Islamic terrorists' plans to attack the West.
Colleen LaRose -- also known as Jihad Jane -- pleaded not guilty to conspiring with terrorists during her arraignment in federal court in Philadelphia.
The FBI said it has evidence LaRose was connected with Islamic radicals online who convinced her to travel to Europe to carry out a terrorist attack.
"She's someone who absolutely frightens every American," said Skip Brandon, the FBI's former deputy assistant director. "She also represents a tremendous threat in terms of intelligence and law enforcement because there's nothing that makes her stand out. She's your worst nightmare."
LaRose allegedly was on a mission to murder a Swedish cartoonist who offended Muslims by drawing unflattering picture of the Islamic Prophet Muhammad.
LaRose was arrested in the United States and could face a penalty of up to life in prison.
LaRose apparently led a troubled life, at one point trying to commit suicide after her father died.
The FBI claims she was willing to become a martyr, writing to radicals online about the plot.
"I will make this my goal till I achieve it or die trying," she wrote in one post.
In a Chicago courtroom hundreds of miles away, American David Headley pleaded guilty to being the advance man for terrorists who bombed and shot their way through several parts of Mumbai, India, two years ago. The attacks left 164 people dead.
Headley, who worked at a local travel agency, admitted today that he helped plan the attacks. He also admitted he was working with al Qaeda on a plot to chop of the heads of employees at a Danish newspaper that had published controversial cartoons of the Prophet Muhammad.
Headley now faces life in prison.
Headley's case and other recent cases, including LaRose's, represent a potentially lethal new threat -- Americans and people with U.S. visas working directly with Islamic radicals, sources told ABC News.
Their cases have raised fears about homegrown terrorists in the United States who may be difficult to spot.
Both Americans allegedly met terrorists on the Internet and aided them in their attacks. Both also were planning missions to murder European cartoonists who had drawn the Islamic Prophet Muhammad, which is forbidden by Islam.
"In my mind, Jihad Jane represents what all my old colleagues were afraid might happen, and is now their worst nightmare," Brandon said, "people who are Americans, not identifiable in any way and can move across borders at will as a terrorist. It's frightening."