Twenty-one residents were displaced and a firefighter was injured after a 4-alarm fire in Dundalk late Tuesday. Baltimore County crews were still on scene Wednesday morning boarding up the affected homes. Most of the damage is out back. The fire broke out in the 100 block Highshire Court around 10:30 p.m. When the first crews arrived they saw heavy smoke coming from the back of the four homes. A Baltimore County firefighter was also injured. Officials said he was on the roof when he fell partially through. He was able to get back up on his feet, evaluated at the scene and was not seriously hurt. One homeowner was also treated at the scene. The American Red Cross will help the residents displaced, while fire investigators try to determine what caused the blaze.