A statement by the prime minister's media office said al-Kadhimi held an outlawed militant group responsible for the assassination and pledged to pursue the killers.
"We pledge to pursue the killers to bring them to justice, and we will not allow the assassinations to return to Iraq to undermine security and stability," the statement quoted al-Kadhimi as saying.
The Iraqi leader described al-Hashimi as an opinion-maker and a supportive voice for the Iraqi forces during the war against the Daesh militants, according to the statement.
Earlier in the day, an Interior Ministry source told Xinhua that al-Hashimi was shot dead in the evening in Zaiyouna district in eastern Baghdad when gunmen on two motorcycles opened fire at his car near his house.
Al-Hashimi received more than 20 bullets and was transferred to a hospital where he was announced dead, the source said.
Al-Hashimi, who was specialised in the affairs of the Sunni and Shiite armed groups, is known for his support for the anti-government protests that broke out last October and his strong criticism of armed groups and militias.
No group has claimed responsibility for the attack yet.
The security situation in Iraq was dramatically improved after the Iraqi security forces fully defeated the extremist Daesh militants across the country late in 2017, but sporadic attacks occurred in the war-ravaged country.
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