DENVER -- A group of serial bank robbers has struck again, for the fourth time in the last month.

On Wednesday night, the FBI-dubbed "Hopping Hooded Bandits" robbed the Bank of Denver, located at 800 E. 17th Ave., in their typical fashion.

Surveillance photos show a group of three to four men walking into the bank wearing hoods and masks and brandishing their weapons.

Investigators said the take-over style robbery is the most dangerous because if anything goes wrong it can quickly turn violent.

FBI spokesman Dave Joly said that's why the reward for information is now $25,000, and they are anxious to get answers before another heist.

"Someone has to know that they're doing this. We're asking them to come forward, accept the reward money, and tell us about the suspects," said Joly.

Investigators dealt with a string of robberies Wednesday -- four banks in the Denver metro area were hit in unrelated robberies.

Joly said that it is common for heists to go up during the holiday season.

If anyone has any information on these robberies, contact the FBI Rocky Mountain Safe Streets Task Force at 303-629-7171; or, you can remain anonymous by calling CRIMESTOPPERS at 720-913-STOP (7867).