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Dramatic images show fire erupting inside Denver's Mile High Stadium

The fire department said the blaze was in the third-level seating area at the home of the NFL’s Broncos .

Authorities are investigating a fire that broke out Thursday at Denver's Mile High Stadium, which quickly scorched a large section of the facility and sent a plume of smoke over the city, officials said.

Denver Fire Department spokesman Greg Pixley told reporters that no injuries had been reported and that the source of the blaze remained unclear.

Seventy-five firefighters responded and extinguished the flames by Thursday afternoon, although they were still working to ensure it hadn't spread to other parts of the stadium, he said.

Pixley said the fire appeared to have grown to about 1,000 square feet.

"We’re lucky to have had the capabilities to get ahead of the fire, and we’re lucky the sprinkler system started to hold the fire in check," he said.

The fire broke out shortly after 2 p.m. local time. Responding firefighters saw black smoke pouring over the side of the downtown stadium, the second stadium named Mile High in Denver, which was built in 2001, Pixley said.

NBC affiliate KUSA reported that a smoke plume could be seen across the city.

The fire department, which published several images showing firefighters battling large flames, said the fire scorched the third-floor seating and suite area.

The stadium said it occurred in a construction zone. Pixley said the plastic seats in the section that burned did so with "significant ferocity" because of the synthetic plastics used in their construction. He noted that plastic seats in the old Mile High Stadium burned in 2002 when it was being torn down.