Prominent Iraqi security expert Husham Al Hashimi was shot dead on Monday night by unknown gunmen outside his house in Baghdad.
Interior Ministry sources said Al Hashimi was taken to hospital where he died shortly after the shooting.
A medical source at the hospital said he suffered "bullet wounds in several body parts".
Videos that appeared to depict the attack on Al Hashimi, 47, showed several figures on motorbikes pulling up near his hose and waiting for him to return. A few moments later the car reportedly carrying Al Hashimi pulls into the driveway, a man runs over and opens fire from around a metre away from the vehicle before fleeing on a moped with the other men.
Born in Baghdad in 1973, he was a well-respected security analyst who wrote extensively about ISIS and Al Qaeda, including three books, and advised the current and past governments on terrorism and extremist groups.
He was strongly in favour of the popular protests that erupted across Baghdad and Iraq's Shiite-majority south in October, which slammed the government as corrupt, inefficient and beholden to neighbouring Iran.
Al Hashimi had been working closely with Iraqi Prime Minister Mustafa Al Kadhimi and was a member of the Iraq Advisory Council, a panel of prominent experts and former policymakers.
Mr Al Kadhimi vowed to hold the perpetrators to account.
“We vow to search and punish the killers, we will not allow assassination to return to Iraq,” he said on Twitter.
Iraqi President Barham Salih backed the prime minister's statement.
“The least we can do [is to] expose these criminals and bring justice to ensure that security and peace prevails for our country,” Mr Salih said on Twitter.Al Hashimi was popular on Twitter with 147,000 followers. Less than an hour before the news of his death emerged, Al Hashimi commented on the social media platform on the divisions forming in Iraqi politics since Mr Al Kadhimi took office in May.
Many of his followers expressed their condolences and shock, reacting to his final message.
While more than 500 were killed in mass protests that erupted late last year, high-profile political killings have been rare recently.
"Cowards killed my friend and one of the brightest researchers in Iraq, Husham Al Hashimi. I am shocked," wrote Harith Hasan, who was a researcher before becoming an adviser to Mr Al Kadhimi.
Martin Huth, the EU ambassador to Iraq, also mourned Al Hashimi’s death.
“Together with his family and friends, we mourn the death of Dr Husham Al Hashimi," Mr Huth said.
"The perpetrators of this heinous crime must be brought to justice."
The British ambassador to Iraq, Stephen Hickey, condemned the killing.
“Devastated and deeply saddened by the news of the killing of Husham Al Hashimi," Mr Hickey said on Twitter.
"Iraq has lost one of its very best – a thoughtful and brave man. These attacks cannot continue.
“The government, supported by the international community, must hold the perpetrators to account."
The UN's top official in Iraq, Jeanine Hennis-Plasschaert, said she was shocked by the assassination.
“We strongly condemn this despicable act of cowardice," Ms Hennis-Plasschaert said.
"Our heartfelt condolences to his family and loved ones. I call on the government to quickly identify the perpetrators and bring them to justice." – Additional reporting by AFP