A man dubbed the "swine flu bandit" struck two more Chicago banks on Monday
The hold-ups bring the number of banks he's believed to have hit to ten.
Both of Monday's heists were at Chase bank branches.
"Somebody's desperate for money.. compulsive gambler, drug addiction... who knows," said FBI spokesman Ross Rice. "We won't know the true motivation other than money until the person is identified and apprehended."
Of the ten robberies, eight took place in Chicago, one in Oak Lawn and one in northwest Indiana.
Rice said the robberies have gotten "more brazen," saying that the robber initially began his spree with a surgical mask and holding up just one teller and has escalated to "armed takeover robberies" involving multiple tellers.
On at least two occasions he told suspicious bank employees who confronted him when he entered the bank that he was recovering from the swine flu.
"We've never seen his mode of escape. He always disappears from the bank on foot and disappears into the passing crowd," said Rice.
The "Swine Flu Bandit" is described as a white man between the ages of 25 and 35.
He stands between 5-feet, 6-inches and 5-feet, 8-inches tall with a thin build, black hair and brown eyes.
He has worn a cap or hooded sweatshirt and sunglasses during each robbery, and covers his face with a painter's or surgical mask.
The FBI is offering a $10,000 reward for information that leads to an arrest and conviction.