UPDATED 08/09/11 5:18 a.m.

CHICAGO (CBS) — Police continue their search for the gunman who shot a man in the head on the State Street bridge downtown in broad daylight Monday.

As CBS 2’s Susanna Song reports, the 29-year-old man was shot in the head on the west side of the bridge near North State Street and Wacker Drive just after 5 p.m. He was taken to Northwestern Memorial Hospital in critical condition.

LISTEN: WBBM Newsradio’s Lisa Fielding reports

Chicago Police Cmdr. Gary Yamashiroya said the victim was heading to work, northbound on the bridge, when a male subject approached him, pulled out a weapon and shot him in the head.

The incident happened at the height of the evening rush hour, as people were heading home in the train. Witnesses heard a gunshot, and saw a man fall to the ground, covered in blood.

“Two men were together – I thought they were together – and all we see is just one man took out his gun and shoot him in the back of the head,” said witness Sameisio Lewis.

Lewis told WBBM Newsradio’s Lisa Fielding that it appeared the shooter pulled the trigger after a brief conversation with his victim.

“It’s just a shame that, walking down the street you see a murder,” Lewis said.

“I was shocked because I didn’t want anything like this to happen right next to the Trump Tower, or downtown period,” he continued.

When police arrived, the victim was able to talk, telling officers someone had shot him.

“We don’t believe this was as random act. We think that this was an instance where the person was specifically targeted,” Yamashiroya told reporters at a news conference near the shooting scene.

No weapon has been recovered. Police hope video cameras at the busy downtown area will yield more information.

“We are still determining the motive,” Yamashrioya said. “We have several witnesses and we’re also reviewing evidence.”

The State Street bridge over the Chicago River was cordoned off by police investigating the shooting. State Street was blocked off between Wacker and Kinzie.

A man who refused to give his name said he heard a single gunshot coming from the area of the bridge.

The witness said he gave chase on his bicycle when he saw a man running away, but lost him at an intersection.

“I got stopped by the light and I didn’t want to get hit by a car,” the witness said.

At last check, the victim remained in serious to critical condition at Northwestern Memorial.

The suspect is described as a black man about 6 feet tall with a short afro and a skinny build. He was wearing a light blue dress shirt with the sleeves rolled up and black shorts, and was holding a chrome-colored handgun, police News Affairs Officer Darryl Baety said. He might have been wearing a raincoat.

Contribuing: Sun-Times Media Wire © Chicago Sun-Times 2010.