DuShon Monique Brown -- who played Connie on "Chicago Fire" -- died of a blood infection, but it's unclear how her system became contaminated ... TMZ has learned.
The Cook County Medical Examiner's Office tells us Brown's cause of death was sepsis, with an unknown origin. Sepsis typically occurs when harmful bacteria infiltrates the blood, often from an untreated wound.
We're told other contributing factors to her death include high blood pressure and obesity.
As we reported ... DuShon died back in March after falling ill while she was at home in Chicago. She'd just been released from a hospital a few days prior ... after she'd received treatment for chest pains.
She starred on "Chicago Fire" ever since the show debuted in 2012 -- and also had a role on the FOX show "Prison Break" from 2005 to 2007.