Friends and family describe Sharif Mobley as a "good Muslim" who never displayed any violent tendencies. He held down a job and has a wife and a daughter.

But authorities tell a different story. They say Mobley, 26, was recently picked up in Yemen and is a suspected member of the same al-Qaida group that tried to blow up an airliner in Detroit on Christmas Day. And there are more and more Americans joining overseas terrorist groups. According to the Associated Press:

His case surfaces days after charges of terrorist connections were brought against Colleen LaRose, an American-born woman known as "Jihad Jane" who lived for years in Pennsylvania.....U.S. officials fear that Yemen is becoming the next significant terrorist staging ground because of signs that lower-level al-Qaida operatives have been moving into the country, and the Pentagon has proposed spending $150 million to help Yemen battle insurgents....Americans are valuable to terrorist groups, in part because they can travel without arousing much suspicion."The U.S. passport is the gold standard," said Fred Burton, a former U.S. counterintelligence agent who is now a vice president at STRATFOR, a global intelligence company in Austin, Texas.

Mobley graduated from high school in 2002 and organized religious pilgrimages to Mecca. Mobley's former high school friend, Roman Castro, said he knew something was awry when, after returning from the army, Mobley said: "Get the hell away from me, you Muslim killer!"

Mobley's parents deny the charges against their son."Mobley's mother, Cynthia Mobley, told WMGM-TV in Atlantic City, N.J., that her son is "an excellent person who's never been in trouble" and "a good Muslim." As his father, Charles Mobley, pulled out of the family's driveway on the way to see a lawyer Thursday, he said, "I can tell you this: He's no terrorist," the AP reports.

After being arrested in the sweep, Mobley was taken to a hospital, where he took a guard's gun and shot and killed him. He wounded another officer before being subdued. He could face the death penalty under Yemen law.

The lesson to be learned from this is that we need to watch the warning signs. You see it when police discover some serial killer on the news. Neighbors always seem to say he was quiet and smiled a lot, but upon closer inspection, you hear that neighbors ignored the stream of women entering the apartment or the weird smell. Everyone, from Mobley's parents to his imam, say they didn't notice any signs.

"He gave no indication that he would join a group that he's alleged to be a part of right now," Abdel-Hadi Shehata, imam of the Islamic Society of Delaware, told the AP. "I never heard that boy ever talk about shooting anybody, killing anybody."

But the response to his former friend should have been a red flag. I wonder if the friend told anyone. Also, when Mobley moved overseas to study Islam, I wonder if it was sudden and impulsive or had he been thinking about it for a while? Did he cut off all contact with family and friends or did he, as most people in a new place do, reach home for support?

In the case of "Jihad Jane" it seems that she may have been swayed by an online love interest in to her alleged activities. She stayed at home to care for her boyfriend's sick father, and again, no one noticed anything.

"She was a good-hearted person," her former boyfriend said. "She pretty much stayed around the house."

If we pay attention to the signs, maybe we can prevent a tragedy.