CHICAGO — A 17-year-old was shot in the buttocks in Chinatown while robbing a woman Tuesday — and police said the shooter was a witness to the stickup.

It happened at 10:15 a.m. Tuesday as a 25-year-old woman was walking in the 2100 block of South Tan Court in Chinatown.

Two people walked up to her and one of them took out a gun and grabbed the woman's purse, police said.

The two ran away, but a man who saw the robbery shot at them, police said.

Just a few blocks away, officers responding to the robbery saw a suspect, 17, and chased him, police said. They arrested the teen in the 100 block of West Cermak Road.

At 11:12 p.m., the other person, who had been shot in his buttocks, told officers he'd been wounded in the 3400 block of South State Street. He was taken to Northwestern Hospital, where his condition was not available but he was listed as "stable," police said.

Officers found the teen, 17, had a gun that matched the description of the gun used in the robbery and arrested him, police said.

The teens were charged with armed robbery, and the teen with a gun was charged with unlawful use of a weapon, police said.

Officers have not identified the man who shot the robber — but they are interested in talking to him.