A four-alarm fire broke out at an apartment building in Chelsea on Wednesday morning. 

Officials said that a 3-year-old boy was trapped by the fire, but was able to escape when crews went in to help, and a building occupant was able to get him out of the building.

The massive blaze on Eleanor Street started around 10 a.m., and was under control in about 45 minutes after fire departments from Boston, Cambridge and Medford came out to assist. 

One firefighter fell and broke his left wrist, injured his right wrist and suffered a back injury. He was taken to a nearby hospital. Another firefighter suffered an injured shoulder, but was able to remain on duty.

The fire started on the third floor of the 24-unit apartment building and spread to the fourth floor. In total, 10 to 12 units were heavily damaged from smoke, flames and water. 

The cause of the fire remains unknown at this time.