At least one person was killed in a lunchtime shooting at a medical building in a busy commercial area of Atlanta, and the suspected gunman was still at large, police said on Wednesday.

Police, who said at least four people were hospitalized after the gunfire, responded to calls about an active shooter just after 12:30 p.m. ET in the Midtown section of the city, the Atlanta Police Department said in a tweet.

Authorities identified the suspected shooter as 24-year-old Deion Patterson and said he was armed and dangerous.

Police posted four photographs of the suspect on Twitter. They showed a man in a doorway, his arm raised and pointing what appeared to be a handgun. His target was not visible in the photos.

The suspect had a mask over his face and nose and was wearing a sweatshirt and carrying a brown backpack.

“Officers are actively searching for the suspect and any other victims,” the department said.

The four injured people, all of whom were adults, were taken to Grady Memorial Hospital. Three victims were in critical condition and undergoing surgery while the fourth was being treated in the emergency room, the hospital’s chief medical officer, Robert Jansen, said during a news conference.

Atlanta Mayor Andre Dickens advised people in the area to shelter in place, and police cordoned off some streets in Midtown.

About an hour after the gunfire, a SWAT team gathered near the High Art Museum, about three-quarters of a mile (1.2 km) from the shooting scene, according to a video posted on Twitter by a local CBS affiliate.

Atlanta Public Schools said several schools in the area would operate on external lockdown for the rest of the day as a precaution.