MOSUL, Iraq (AFP) - Four civilians were killed and 35 people injured on Saturday by a car bomb in a Shia district in the restive Iraqi city of Mosul, a police officer told AFP. Violence has dropped markedly throughout Iraq in recent months, but attacks remain common, especially in Baghdad and Mosul in the north. Meanwhile, a truck driver was mistakenly killed by US forces outside Tikrit, the US army said on Saturday. The troops had stopped along a highway between Tikrit and Balad after one of the vehicles in their logistics convoy broke down early Friday. They signalled for the truck, which was approaching the convoy from the rear, to stop but it sped up as it closed in, according to a statement. A soldier then fired on the truck after the driver did not respond to the warnings, killing him. A teenage passenger was unharmed, and the incident is under joint US-Iraqi investigation. We regret the loss of life and our prayers are with the family of the driver, said Major Derrick Cheng, spokesman for Multi-National Division North. Its a tragic event, but the soldier perceived a threat. He was protecting the convoy against what he thought was an attack and did so in accordance with the security agreement, Cheng added. He was referring to a US-Iraq security accord that led to the pullout of US combat troops from Iraqi cities in June, and calls for American forces to leave the country by the end of 2011.