Two plain-clothes Chicago police officers were shot and wounded as they sat in an unmarked vehicle in what authorities are describing as a targeted attack on the city's South Side. A manhunt was underway on Wednesday. Two vehicles pulled up alongside the officers and opened fire Tuesday evening in the high-crime Back of the Yards neighborhood. One officer was shot in the arm and hip and the other in the back, police spokesman Anthony Guglielmi said. The officers were conducting a follow-up investigation to a previous incident, but Guglielmi said they were wearing civilian clothes with vests bearing police badges. Johnson said their vehicle was unmarked. "We believe that the officers were definitely targeted," Guglielmi said. Meanwhile in an unrelated case, the US Department of Justice has reportedly decided not to charge two white officers who shot and killed a black man in Louisiana last summer. Video footage appearing to show the officers holding down Alton Sterling as they fired their weapons sparked days of protests in Baton Rouge. News of the decision leaked to US media on Tuesday before the city mayor or the Sterling family had been told. People soon began gathering outside the shop where the incident happened. A vigil was organised for Tuesday night, with small crowds gathering near the city's police headquarters. The civil rights investigation was opened soon after the 37-year-old was killed outside the grocery shop he was selling CDs. At the time, a series of fatal police shootings involving African-Americans had sparked a debate about police use of force. The federal decision not to prosecute the two officers comes with a new US government and a new head of the Justice Department, Attorney General Jeff Sessions. It does not mean the state of Louisiana could not bring its own charges. Police were called after reports of a man threatening people with a gun outside a shop. Mobile video footage appeared to show two officers wrestling a man in a red shirt to the floor. One of the officers pinned the man's arm to the floor with his knee and then appeared to pull out his gun and point it at the man. A voice is heard shouting: "He's got a gun!" Then shots ring out and the camera moves away. — Agencies