A police officer is among those injured in a mass shooting at Mercy Hospital in Chicago.

CBS News reports that police were responding to what they described as an “active shooter situation” on the hospital’s campus in the city’s south side.

Chicago Police spokesperson Anthony Guglielmi reportedly said on Twitter that an officer had been critically injured and was receiving treatment. A witness told the news station that he saw a police officer arrive to the scene in an SUV before a gunman shot at him. According to the witness, the gunman reloaded, fired again at a person who was on the ground, and went into the hospital.

It is unclear if the active shooting is continuing, or where is it taking place. This is a developing story and all information is preliminary. CrimeOnline will provide further updates when more information is available.