Police were searching for a lone gunman spotted outside the store by a witness, but said they had not identified him.

The victims were all found in the back of the Lane Bryant women's clothing store in an area not frequented by customers, Tinley Park Police Sgt T.J. Grady told reporters.

It appeared to be a robbery that 'went rather poorly,' he added.

Sgt Grady declined to say whether the victims were customers or employees of the store, located at the Brookside Marketplace shopping centre in a southwestern suburb of Chicago.

Police said they received a call at 10:44 am (12.44am Sunday Singapore time). CST reporting multiple gunshot victims inside the store.

The suspect had fled and they issued an alert for a man wearing black jeans, a black waist-length winter coat and a black knit cap, police said. -- REUTERS