One person died and at least 11 were injured in a release of poisonous hydrogen sulfide gas at an apartment building near the University of Texas at Austin on Wednesday, fire officials said.

Austin firefighters responded to a hazardous materials call from an apartment at the complex and found a man believed to be in his 20s dead at the scene, said Austin Fire Department Lt. Kevin Haas.

Haas said the initial investigation determined that the release of the chemical was deemed "an intentional act," and was being investigated as a suicide.

It was unclear where the gas came from, although Haas said the victim likely released it from a container brought into the apartment.

Hydrogen sulfide is a colorless gas that smells of rotten eggs.

Six people were taken with non-life-threatening injuries to a hospital, and five others refused care, Austin-Travis County Emergency Medical Services spokesman Captain Darren Noak said.

The Austin Fire Department responded around 2:30 p.m. to a call reporting a cardiac arrest. Fire department personnel found a handwritten note on a closet door inside the apartment where the deceased victim was found that warned of danger, and to "get out," and of the presence of hydrogen sulfide, Haas said.

The emergency responders decontaminated the building and evacuated residents. Photos on social media showed emergency workers and vehicles including a fire truck on a street that had been blocked off from traffic.

Noak said residents were later cleared to return to the building.

It was unclear whether the man who died or the people injured were students, a University of Texas at Austin spokeswoman said.