"Serial bank robbers with this amount of bank robberies are not common," FBI Special Agent Kathy Wright said.

One man is nicknamed the "Shaggy Bandit" because investigators think he looks like the character "Shaggy" from "Scooby Doo" cartoons. He is accused of robbing 13 banks from Longmont to Boulder to Denver to Aurora. He's also linked to a bank robbery in Flagstaff, Arizona.

"He is very low key. He doesn't draw any attention to himself," Wright said. "He'll approach the victim teller. Usually, he has a backpack and he'll put it on the counter."

The other bank robber is dubbed the $83 Bandit because investigators say he asks for money in unusual dollar amounts. Investigators believe he is homeless or frequents homeless shelters. The FBI says the $83 Bandit has committed eight robberies in Wheat Ridge, Arvada, Denver and Aurora.

The FBI believes the serial robbers are not connected to each other, but they are worried that each one is becoming more and more brazen.

"We don't know if they're armed and we don't know what they would do if confronted," Wright said. "So we always consider them armed and dangerous."

Both men have now started robbing the same bank more than once, according to the FBI.

The Shaggy Bandit has struck a US Bank twice on Colorado Boulevard in Denver. The $83 Bandit is accused of robbing the same Vectra Bank on Speer Boulevard in Denver two times in a month.

Bob Pence is a former FBI special agent in charge. He has worked numerous bank robbery cases in his career. He says it is not that unusual for robbers to hit a location multiple times.

"When you do a bank robbery and you think you're gonna be successful and you get away with it temporarily, you have a certain comfort level to think well, nothing happened here," Pence said. "The more robberies a bank robber commits, the more he's going to feel hungry to do more."

But Pence says statistics show these bank robbers will eventually be caught.

"In bank robberies, the solution rate is higher than most crimes," Pence said. "It's a national problem right now. Bank robberies are up across the country."

So far this year, there have been 123 bank robberies in Colorado. In all of last year, there were 150 bank robberies. The FBI believes at this rate, the state is on pace to top last year.

If you know anything about these cases, please call the FBI at 303-629-7171. That will connect you with the Rocky Mountain Safe Streets Task Force.