A fire caused damage at Denver Broncos’ Mile High Stadium on Thursday in Denver, Colorado. Numerous videos were shared on social media capturing billows of smoke coming from inside the complex.

The Denver Fire Department said that flames burned in a construction zone on the third level. At least six rows of seats in two sections were heavily damages.

The fire occurred near the East Club Lounge and that part of the stadium was evacuated. No injuries were reported in the incident. The cause of the fire remains unknown.

DFD said that the sprinkler system was working but it was unable to combat the size of the blaze.

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“This type of plastic that the seats are composed of, burns with – not only significant ferocity – very, very large flames, but also produces a tremendous amount of black smoke due to the synthetics and petroleum products that are used,” Fire Capt. Greg Pixley said. “That meant that that fire was going to need a great deal of help from our firefighters.”

Pixley also said that the fire burned what is estimated to be at least 1,000 square feet, according to CNN.

At least 100 people are estimated to be at the second level for an event which was underway at the time. 

Pixley said, “Historically, we had a very similar fire when we tore down the old Mile High Stadium where the plastic seats caught on fire. So our firefighters have an experience with this type of fire fight.”

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Twitter user Kate Braboy said she was on a tour of the stadium when the fire broke out.

The stadium, formerly known as Invesco Field and Sports Authority Field, opened in 2001 next to the site of the previous arena named Mile High Stadium.

The Broncos are not currently playing in the stadium, but there are several shows expected to take place in the coming months. Luke Combs, Red Hot Chili Peppers, The Weeknd and Foo Fighters are all set to perform at the stadium.