The latest chapter in the FBI's war on terrorism began in a Circuit City store in Mount Laurel 16 months ago.

A clerk called local police and said someone had brought a videotape to the store to be duplicated. The clerk watched it and found it "disturbing."

It showed 10 young men in their early 20s "shooting assault weapons . . . and calling for jihad."

The Mount Laurel police called the FBI. And that, authorities said yesterday, is how the FBI began to investigate a plot to kill soldiers at Fort Dix.

While authorities declined to publicly identify the clerk or his company yesterday, they went out of their way to call him a unsung hero.

"This person did what we want all citizens to do," said U.S. Attorney Christopher J. Christie. Others familiar with the matter confirmed the store where the case began.

Workers at the store declined to comment. A Circuit City spokesman, Jim Babb, said he couldn't confirm his company's involvement, but added: "I can tell you that our company policy is to cooperate with authorities."