An off-duty Chicago police commander was shot dead while helping other officers to make an arrest.
Commander Paul Bauer was killed while trying to help other members of a tactical team in “apprehending a suspicious person”, The Chicago Police Department confirmed said.
A spokesman called a it a “senseless murder”, adding that person of interest was in custody.
“It’s a difficult day for us”, Chicago Police Department Superintendent Eddie Johnson told reporters.
A 31-year veteran of the force, Mr Bauer left behind a wife and daughter, officials said.
The shooting occurred at the Thompson Centre, a building in downtown Chicago that hosts government offices.
Mr Bauer died after being transported to the hospital.
“The hearts of every Chicagoan are heavy as we mourn the loss of Commander Paul Bauer”, Chicago Mayor Rahm Emanuel said.
Gun violence has long plagued Chicago, America’s third-largest city.
The city saw2,785 shooting incidents affecting nearly 3,500 victims in 2017, according to police statistics, along with 650 murders.
But both of those figures represented decreases from the previous year.