An aide to Iraq's top Shia cleric, a police officer and his father were also shot dead in separate attacks.
Oil Ministry employee Ali Hamid Alwan al-Dulaimy, 31, walked out of his house to his car when three men firing pistols from a KIA minibus killed him, his brother, Ahmed Hamid Alwan al-Dulaimy, said in a telephone interview.
The slain engineer was killed in Kazimiyah, northern Baghdad, before trying to go to Baghdad University, where he was studying, his brother added.
In Samarra, 95km north of Baghdad, armed men assassinated police officer Omar Majeed Shakir al-Dosh and his father on Thursday, said police Lieutenant Qassem Mohammed.
Also on Thursday, two police officers were killed and three people were wounded, one of them a police officer, in a bomb explosion in Baquba, about 60km northeast of the capital, police said.
On Wednesday, armed men shot dead Shia cleric Mohammed Tahir al-Allaq, representative of Grand Ayat Allah Ali al-Sistani in the Jurf al-Nadaf area near al-Madain, about 20km southeast of Baghdad, police Lieutenant-Colonel Shakir Wadi Hamoud said.
Hamoud said al-Allaq, imam of al-Jawadain Mosque in the predominantly Shia al-Baladiyat neighbourhood of New Baghdad, was killed while driving to Kut, a city 160km southeast of Baghdad.
Two police officers were killed in
the Baquba attack
Armed men fired at the cleric's car, forcing it off the road into a ditch, Hamoud said. Al-Allaq was shot in the head and police recovered his body after five hours, Hamoud added.
Hundreds of mourners staged noisy protests denouncing what they called terrorism at al-Allaq's funeral on Thursday in Baghdad's impoverished Shia district of Sadr City, his brother Qassem al-Allaq said.
Fighters have been targeting religious leaders, government employees and police officers in a campaign aimed at disrupting Iraq's US-backed reconstruction.