Reuters/Tikrit, Iraq

On April 1, the city of Tikrit was liberated from the extremist group Islamic State. The Shia-led central government and allied militias, after a month-long battle, had expelled the  Sunni radicals.

Then, some of the liberators took revenge.

Near the charred, bullet-scarred government headquarters, two federal policemen flanked a suspected Islamic State fighter. Urged on by a furious mob, the two officers took out knives and repeatedly stabbed the man in the neck and slit his throat. The killing was witnessed by two Reuters correspondents.

The incident is now under investigation, interior ministry spokesman Brigadier General Saad Maan told Reuters.

Since its recapture two days ago, the Sunni city of Tikrit has been the scene of violence and looting. In addition to the killing of the extremist combatant, Reuters correspondents also saw a convoy of Shia paramilitary fighters - the government's partners in liberating the city - drag a corpse through the streets behind their car.

Local officials said the mayhem continues. Two security officers, speaking on condition of anonymity, said on Friday that dozens of homes had been torched in the city. They added that they had witnessed the looting of stores by Shia militiamen.

Later Friday, Ahmed al-Kraim, head of the Salahuddin Provincial Council, told Reuters that mobs had burned down "hundreds of houses" and looted shops over the past two days. Government security forces, he said, were afraid to confront the mobs. Kraim said he left the city late Friday afternoon because the situation was spinning out of control.

"Our city was burnt in front of our eyes. We can't control what is going on," Kraim said.

Those reports could not be immediately confirmed.

It wasn't supposed to be this way. Islamic State, an Al Qaeda offshoot that arose from the chaos in Iraq and Syria, slaughtered thousands and seized much of northern and central Iraq last year. The government offensive was meant not only to dislodge the group but also to transcend the fundamental divide in fractured Iraq: the enmity between the now-ruling Shia majority and the country's formerly dominant Sunni minority.

Officials close to Prime Minister Haidar al-Abadi, a moderate Shia, had described the Tikrit campaign as a chance to demonstrate his government's independence from one source of its power: Iraqi Shia militias backed by Shia Iran and advised by Iranian military officers. Sunnis deeply mistrust and fear these paramilitaries, accusing them of summary executions and vandalism. But Abadi has had to rely on the Shia militias on the battlefield, as Iraq's regular military deserted en masse last summer in the Islamic State onslaught.

The militia groups spearheaded the start of the Tikrit assault in early March. But after two weeks of fighting, Abadi enforced a pause. Asserting his power over the Shia militias, he called in U.S. airstrikes.

Now, the looting and violence in Tikrit threaten to tarnish Abadi's victory. It risks signaling to Sunni Iraqis that the central government is weak and not trustworthy enough to recapture other territory held by Islamic State, including the much larger city of Mosul. Tikrit, hometown of the late dictator Saddam Hussein, is in the Sunni heartland of Iraq.

At stake is much more than future votes: Islamic State's rapid conquests in 2014 were made possible by support from Sunni tribal forces and ordinary citizens. They were convinced that the government - under Abadi's predecessor, Nuri al-Maliki - viewed their community as terrorists. If Sunnis dislike what they see in Tikrit, they may not back the government's efforts against Islamic State.

Defending lives and property

On Friday, the government sought to assure all sides that it will enforce order. Abadi issued a statement calling on the security forces to arrest anyone breaking the law.

Asked to comment on the scenes witnessed by Reuters, his spokesman Rafid Jaboori said he would not address individual incidents but said: "People's lives and property are priorities, whether in this operation or in the overall military effort to liberate the rest of Iraq."

Sunni lawmakers who visited Tikrit complained that events have spun out of control since the security forces and militias retook the city.

Parliamentarian Mutashar al-Samarrai credited the government with orchestrating a smooth entrance into Tikrit. But he said that some Shia paramilitary factions had exploited the situation. "I believe this happened on purpose to disrupt the government's achievement in Tikrit," Samarrai said. "This is a struggle between the (paramilitaries) and the government for control."

Neighborhoods entered by the Iraqi forces and Shia paramilitaries have been burnt, including parts of neighboring Dour and Auja, the birthplace of Saddam Hussein.

Security forces blame Islamic State for rigging houses with explosives, while Sunnis suspect the Shia militias and the army and police of deliberately torching their homes.

Looting has also been a problem. Shia paramilitary fighters in pickup trucks raced through the city carrying goods that appeared to have been looted from homes and government offices.

The vehicles were crammed with refrigerators, air conditioners, computer printers, and furniture. A young militia fighter rode on a red bicycle, gleefully shouting: "I always dreamed of having a bike like this as a kid."

Brigadier General Maan, the main spokesman for the government forces, said police were stopping vehicles that appeared to have stolen items. "We are doing our best to impose the law."

Iran's fingerprints

Passions were running high among the Shia militia groups before the assault. Islamic State beheaded people and carried out other atrocities in the lands it conquered. In particular, the militias wanted revenge for Islamic State's killing in June of hundreds of Iraqi soldiers captured from Camp Speicher, a base near Tikrit. It was an event that came to symbolize the Sunni jihadists' barbarism.

Despite Baghdad's efforts to rein in the paramilitaries, the fingerprints of the Shia militias - and of Iran itself - were all over the operation's final hours.

On Wednesday, as Tikrit fell, militiamen were racing to stencil their names on houses in order to take credit for the victory.

An Iranian fighter, with a Kalashnikov rifle slung over his shoulder and a picture of Iranian Supreme Leader Ayatollah Ali Khamenei pinned to his chest, bragged about Tehran's role in the campaign.

"I am proud to participate in the battle to liberate Tikrit," said the man, who called himself Sheik Dawood. "Iran and Iraq are one state now."

On the edge of Tikrit in the hours after the city's fall, a Shia paramilitary group drove in a convoy past several police cars.

The militiamen had strung the corpse of a suspected Islamic State fighter from the back of a white Toyota pickup truck. The cable dragging the man snapped, and the vehicle stopped.

The men got out to retie the bullet-riddled corpse. As they fastened the cable tighter to the body, a song about their victory over Islamic State played on the truck's stereo. Then they sped off, the corpse kicking up a cloud of dust.

The policemen standing nearby did nothing.

On Wednesday afternoon, Reuters saw two suspected Islamic State detainees - identified as an Egyptian and a Sudanese national - in a room in a government building. The Egyptian and the Sudanese were then taken outside by police intelligence.

Word spread that the two suspected Islamic State prisoners were being escorted out. Federal policemen, who had lost an officer named Colonel Imad the previous day in a bombing, flocked around the detainees.

The interior ministry spokesman, Brigadier Maan, said the Egyptian had stabbed an Iraqi police officer, which explains the anger against him. Reuters couldn't verify that claim.