Man questioned over deadly blast shot dead his interrogator, interior ministry says.
Twin car bombs, detonated within minutes of each other, killed two people in a second attack in Ramadi.
Among the dead was a policeman. Four other people were hurt.
The devices had been left in cars parked in the visitors’ car park at the Traffic Police Department.
Earlier in the day, three mortar rounds slammed into the commercial district of Baghdad, killing one.
Last week twin suicide bomb attacks in central Baghdad killed 153 people and wounded about 500 more. The strikes constituted the deadliest day for the country in two years.
The violence comes before parliamentary elections due on January 16.