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Hostilities have flared as Iraq reels from the assassination of , 47, who was gunned down by unknown assailants on motorbikes outside his home on Monday. He had received death threats from the Islamic State group and Iran-backed militia .

Hashimi's killers are still unknown but many point to the timing of the assassination, coming just two weeks after a raid on the headquarters of the militia south of Baghdad.

They speculate Hashimi may have fallen victim to escalating tensions between the government and militia groups. Days before his death a study he wrote on the inner workings of Iran-backed militia groups in Iraq was published.

US Secretary of State Mike Pompeo has also linked Hashimi's death to Iran-backed militants.

Prime Minister Mustafa al-Kadhimi has vowed to avenge the killing, but a serious investigation risks further stoking tensions with Iraq's militias. Kataeb Hezbollah's spokesman, Mohammed Mohie, speaking to The Associated Press on Wednesday, described the raid on his group's headquarters as an act of "provocation".

Kadhimi has said that bringing armed groups under state control is a goal of his administration.

Iraqi security forces descended on Kataeb Hezbollah's headquarters in the Dora neighborhood of Baghdad on June 26 and rounded up 14 men suspected of orchestrating rocket attacks against US forces based at the Baghdad airport, and the US Embassy.

But days later, 13 of them were released after investigators said they could only link one of them to the attacks. Rocket attacks then resumed, targeting the heavily fortified Green Zone where the US Embassy is located, and the airport.

"We have our conviction that these provocations (by Kadhimi) will not stop, and will continue and there will be an escalation," said Mohie. "That will be in the next stage."

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Kataeb Hezbollah is part of the Popular Mobilisation Forces, the state umbrella group encompassing an array of militias including Iran-backed ones. The US has blamed the group for rocket attacks on its embassy.

Kadhimi came to power with nods from both Iran and the US, and balancing relationships between both rivals was expected to be a chief challenge for his government. 

In response to continued rocket fire, the US Embassy installed a C-RAM system designed to intercept the projectiles. Mohie said the move was "another provocation" that effectively turned the embassy into a "military base".

Meetings were held shortly after the US Embassy began testing the C-RAM system, a senior Shia political official said, and a decision was made to ramp up pressure on the premier. The official spoke on condition of anonymity in line with regulations.

"We have great concern that Kadhimi plans to liquidate these (militant) factions," he said.

Hours after the Dora raid, armed men in pickup trucks entered the Green Zone and surrounded an office of the elite Counter-Terrorism Forces, the security force that had carried out the operation.

"It showed the prime minister the limits of his authority and that to deal with this will require a significant amount of effort," said Iraq analyst Sajad Jiyad.

Hashimi, who supported al-Kadhimi and served as an advisor to previous premiers, had written a study looking into the extent of Iranian influence inside the PMF. It was published July 1, days before he was shot dead.

In it, he found that a disproportionate number of Iran-backed factions held leadership and advisory positions within the PMF. Of the 67 Shia factions within the group, he found that 44 were followers of Iran's Ayatollah Ali al-Khamenei.

The question now is what can Kadhimi do next.

"It's almost like the militias are saying, if you want an open confrontation bring it on," said Iraq researcher Fanar Haddad. "What is the face-saving alternative? It's not clear."

US Secretary of State Mike Pompeo on Wednesday demanded justice for Hashimi's killing, saying his death came in the wake of repeated threats by Iran-backed armed groups.

"The United States joins partner nations in strongly condemning his assassination and call(ing) for the government of Iraq to bring to justice the perpetrators of this terrible crime... swiftly," he told a news conference in Washington.

Some experts have voiced fear of a new violent phase in Iraq and believe the turning point may have come in January when a US strike in Baghdad killed a top Iranian general, .

President Donald Trump's administration has sought to check Iran's regional activities and choke its economy and frequently seeks to throw a spotlight on purported nefarious activities backed by the clerical state.

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