CHICAGO (CBS) — A Chicago firefighter was injured early Friday morning while responding to a small fire at U.S. Cellular Field.

The fire started at about 2:25 a.m., about 11 hours before the gates open for the White Sox home opener against the Indians, according to the Fire Department.

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The fire started in a storage area on the first floor inside the stadium near Gate 3 behind first base. The sprinkler system kicked in and kept the fire from spreading.

“We had our overnight crew here. They were alerted by the alarm system that there was a fire in our warehouse, which was contained by the sprinkler system. The Chicago Fire Department arrived, extinguished the fire completely, and now we’ll be ready for baseball,” White Sox spokeswoman Sheena Quinn said.

Firefighters put the fire out around 3 a.m.

One firefighter was taken to Mercy Hospital and Medical Center for treatment. He was taken out of the stadium on a stretcher, and was wearing an oxygen mask. Sources said he suffered heat exhaustion. His injuries were not considered life-threatening. No other injuries were reported.

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The White Sox use the room where the fire started to store promotional items and other paper goods.

“It’s got our extra promos, giveaways, caps, posters, things of that nature. It’s just a facility essentially where we store our extra stuff,” Quinn said.

The Fire Department was using large fans to vent smoke from the inside of the stadium.

The White Sox said Friday afternoon’s game would go on as planned.

“We’re just happy that everyone is okay, and that we’ll be ready for baseball,” Quinn said.

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The cause of the fire was under investigation. The Fire Department’s Office of Fire Investigations and the Chicago Police Bomb and Arson Unit were investigating, but police said there was no immediate indication the fire was intentionally set.