He dressed in military fatigues and flashed official-looking Air Force identification that said he was Tech Sgt. Kenneth Stewart, Williams said.
But detectives learned Stewart was really Isaac McCrumby, 29, an ex-con who operated an Arlington recording studio, police said.
McCrumby now faces federal fraud charges.
"Just dressing up as a person in the military, people would assume a military person would be a credible person, and he would use that ruse to pass the checks,” Williams said.
In a raid on his Arlington home and recording studio, investigators said they found bogus checks and the electronic gadgets he used to make his own credit cards. They also found a fully loaded AK-47.
McCrumby stole more than 4,000 bank account numbers with the help of someone who worked inside a bank, police said.
An investigation into several of his accomplices is ongoing.
For Williams, the investigation was personal.
"My son is in the military,” Williams said. “He's been to Iraq twice. He laid his life on the line. And I'd hate to have someone of that character come and besmirch -- to me it besmirches everybody who's in the military."
McCrumby has a prior criminal record that includes theft in Texas and Arkansas, according to public records.