Police often say they're fortunate so many of the world's criminals are stupid. And stupidity does abound in the case of the alleged terrorists in Cherry Hill.

To wit, the FBI says the suspects practiced for their jihad against Fort Dix by playing paintball. They recorded video of themselves firing guns and shouting "God is great!" in Arabic, then turned over the ominous recording to a clerk at Circuit City in Mount Laurel to make a DVD. They apparently didn't know that federal agents were watching their house for months, a development obvious to worried neighbors on the quiet street.

There's so much stupidity alleged that you might be tempted to chuckle at the suspects' ineptitude - a case of "Jihadi Jerks in Jersey." But that doesn't mean this reputed terrorist cell was harmless, or that law-enforcement officials had an easy job. Assuming that the allegations are proven to be true, there was good police work and appropriate suspicion by people who could have looked the other way.

Start with the clerk at the electronics store. While the video sounds alarming, there have been more obvious examples of threats to U.S. security in recent years that fell through the cracks of law enforcement. This store employee did the right thing by contacting local police, who brought in federal officials. That one phone call got the investigation rolling.

Then the FBI infiltrated the group, recorded the conversations of suspects, and built a case for more than one year. Officials arrested the young men earlier this week without anyone being harmed. Their backgrounds seemed so utterly benign. Three brothers - Shain, Eljvir and Dritan Duka - worked as roofers in Cherry Hill. Mohamad Shnewer was a cab driver in Philly. Serdar Tatar worked at a 7-Eleven near Temple University. Agron Abdullahu of Gloucester County was a supermarket baker.

The group's lack of sophistication might help to explain why the alleged plot to kill soldiers in the United States failed. But the fact that such a group allegedly sprouted in Cherry Hill brings home a troubling aspect of the so-called war on terrorism.

Our national focus has been on fighting terrorists abroad "so they won't follow us home." The Cherry Hill gang was already here - three of them having entered the country illegally years ago.

They had no direct contacts overseas to Osama bin Laden nor to anyone else in al-Qaeda, so far as the FBI knows. What they allegedly did share was a bond in a misguided interpretation of Islam, and an enthusiasm for Jihadist rhetoric. If that's the formula for creating "home grown" terrorists, ensuring security within our borders has become more challenging than many of us had imagined.