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DENVER - Buildings specifically designed to keep thieves out and protect the valuable marijuana plants grown inside caught on fire on Tuesday, making it very difficult for firefighters to get to the blaze.

Denver firefighters experienced just such a problem when they were called to a grow operation at 4430 E. Garfield Ave. on a report of smoke and flames visible from the building. The structure actually contained three separate grow operations. The fire started in the ceiling of one of the operations.

"The problem is that it is fortified. They want to protect their product, so there are steel doors, bricked walls and one of the windows is bricked up. It makes it very difficult for us to get into it to get to the fire," John Afsher, assistant chief for the Denver Fire Department, said.

As marijuana-grow operations increase in Colorado, firefighters are being called more frequently to fires at those facilities.

"They use a lot of electricity and large electrical use a lot of times translates into fires," Afsher said.

Investigators were able to determine an electrical fire was responsible for this incident. Firefighters say that the problems they face with grow operation fires just get started when they gain access to the building.

"With grow operations it is a little diced up because what you see from the front you don't necessarily get when you get inside. So to have small plants, medium plants and large plants at first it is a little confusing, but we're getting used to that, and we're getting to know that industry," Afsher said.

Investigators say the unit where the fire started suffered significant damage while the other two connected units had little damage.