At least two victims have died, along with the shooter, following a mass shooting at Mercy hospital, located on the South Side of Chicago, according to a CBS affiliate station.

NBC 5 Chicago has initiated live coverage of the shooting:

An officer is reportedly one of the victims. They are in critical condition but receiving 'excellent care' and is being transported to University of Chicago Hospital. Witnesses also reportedly saw a woman - according to some accounts a nurse - who had been shot. The shooter, described as a white male with an assault rifle, has been killed.

One reporter said this started as a domestic incident.

Latest from @cbschicago - it appears that this started as a domestic incident outside of Mercy Hospital. Shooter shoots girlfriend then makes his way into the hospital. 1 or 2 people then shot inside. At some point gunman shoots police officer. Gunman has been shot. — Janel Forte (@JanelKHOU) November 19, 2018

Police Activity: Reports of shots fired in the vicinity of 26th and Michigan near Mercy Hospital. Avoid area. Heavy police response incoming pic.twitter.com/yHe19SN8SY — Anthony Guglielmi (@AJGuglielmi) November 19, 2018

There was an 'enormous' police response; a SWAT team unit arrived on the scene.

Dozens of hospital staff gathered outside Mercy Hospital’s east entrance. At least 4 helicopters here. Enormous police response. pic.twitter.com/hVO3jYbvq9 — Sam Charles (@samjcharles) November 19, 2018

#BREAKING: A police officer is in critical condition after being shot during active shooter incident at Mercy Hospital in Chicago. At least one possible offender was shot as well, police say https://t.co/LI1DmEH5Ph #fox5dc pic.twitter.com/keWM9ZFLS9 — FOX 5 DC (@fox5dc) November 19, 2018

One witnessed described hearing an exchange of gunfire. Another witness said he turned around and saw the shooter firing at a victim.

A woman who was in Chicago's Mercy Hospital describes hearing an exchange of gunfire during a reported shooting there: "It was extreme and it was very loud and it was close" https://t.co/mGUPVr9iBP pic.twitter.com/HmuNvMDU6r — CBS News (@CBSNews) November 19, 2018

"I turned around to my right and I see a man shooting someone on the ground": A man who was waiting for results in Chicago's Mercy Hospital describes seeing the active shooter there pic.twitter.com/79GCK88vG4 — CBS News (@CBSNews) November 19, 2018

Another witness told a Chicago TV station that it looked like the shooter was firing at random targets, according to the Associated Press.

Here's a quick rundown of what we know, courtesy of BNO News:

Mercy Hospital in Chicago

Started at 4:18 p.m. ET

Multiple victims, including officer

Possible suspect 'down'

No word on motive

1 officer in critical condition

Video of police streaming into the hospital is already circulating on social media.