FBI agents investigating what caused the rear window of a federal judge's car to shatter have nabbed their culprit - a two-way radio antenna attached to the car's trunk - and have declared the case closed.

When 11th Circuit Court Judge J. Lawrence Edmondson's rear window shattered Wednesday with a loud popping sound, police panicked, thinking the colleague of slain Judge Robert Vance was the victim of an assassination attempt. Federal agents descended on the city, searching for a gunman.Roadblocks were set up, helicopters buzzed the Lawrenceville area and students were locked inside nearby schools for their safety.

The FBI said Friday the window shattered because a two-way radio antenna on the car repeatedly scratched the glass, weakening it.

U.S. Marshals who regularly escorted Edmondson "remembered that whenever they opened the trunk of the car, the antenna would hit the back of the window," said FBI agent Pat Johnson. "That microscopic damage flawed the window. At that moment the harmonics were just right, and that window said, `I've had it. I'm going to collapse."'

Johnson said the case is now closed.