A highway overpass in Fallujah is plastered with Shiite banners, graffiti and posters of militia leaders, a virtual shrine to victory over the Islamic State group in this majority Sunni Muslim city.

The fight to wrest Fallujah from IS control appears to have inflicted considerably less damage to the city's infrastructure than past battles. But scenes like this have the potential to undermine the military's success and hamper the broader fight against IS by reigniting the sectarian tensions that helped fuel the militant group's rise in Iraq.

Fallujah, 65 kilometers (45 miles) west of Baghdad, was declared liberated from the Islamic State group over a week ago. It has mostly been calm since, unlike other parts of Iraq. On Sunday, a devastating truck bombing on a bustling commercial street in downtown Baghdad killed 115 people, brutally underscoring IS's ability to strike the Iraqi capital despite battlefield losses elsewhere.

Once a town made wealthy by trade and industry, after the U.S.-led invasion of Iraq Fallujah became the epicenter of an insurgency against U.S. forces and the militant opposition to the Shiite-dominated central government. While it was under IS control, Iraqi officials repeatedly pointed to Fallujah as a source of the car bombs and other explosives used to attack Baghdad and other areas from the front-line fight.

On 25 June, Iraqi forces retook the city, which had been held by IS since 2014. In a bid to reduce sectarian conflict and prevent abuses, Iraq's military said the government-sanctioned Shiite militias participating in the fight would not enter the center of Fallujah. But days later, Shiite militias known as the Popular Mobilization Forces, or Hashed, were seen walking openly in the streets.

The special forces commander overseeing the Fallujah operation, Lt. Gen. Abdul Wahab al-Saadi, declined to comment on these sightings. But he said the symbolic value of the city for both Iraqis and IS militants made the liberation of Fallujah an especially sensitive operation.

The highway overpass, in particular, has become emblematic of Fallujah's bloody sectarian battles. A year ago, it was the scene of the brutal killing of an Iraqi soldier by IS. Images released by the militants show Mustafa a-Athari, a Shiite from Sadr City, being paraded through town before he was hanged from the overpass as crowds of residents cheered.

Al-Athari was quickly upheld as a martyr, and Iraqi militia leaders pledged to avenge his death, calling on the government to allow them to launch an operation to retake Fallujah. The powerful Iraqi Shiite militia, Asaib Ahl al-Haq, pledged to "crush the skulls" of those responsible.

The event echoed an incident more than 10 years earlier, when an angry mob hanged four American security contractors from a bridge just half a mile (about one kilometer) from the site of al-Athari's murder. The 2004 public killing of the Americans became an iconic image that changed the U.S. public's perception of the Iraq war and preceded a brutal U.S. military response. Thousands of Iraqis and 153 American troops were killed and large parts of Fallujah were virtually levelled.

"This is the city in which some of the worst crimes against humanity have occurred," said al-Saadi, the special forces commander. "It began with the killing of the Americans and continued with the murder of al-Athari."

Small groups of Shiite militiamen fought under the banner of the federal police, but once the operation was declared complete, the militia fighters began raising their own flags. Some Iraqi commanders — speaking anonymously, because they are not authorized to discuss the operation — said the militiamen set fire to houses in the city. The special forces commander said that IS militants set homes alight before their retreat.

The alleged misconduct in Fallujah was small in scale compared to the destruction carried out by Shiite militia participating in the battle for the Sunni-majority city of Tikrit, north of Baghdad.

A spokesman for Asaib Ahl al-Haq, the powerful militia, denied that Shiite militiamen had entered the center of Fallujah and rejected claims that they could destabilize the city.

"We are here for the sake of stability" said Jawad al-Talabawi, who repeated that Shiite groups remained on the city's outskirts. "We gave the blood of our martyrs for the liberation of Fallujah, but do not have any further ambitions."

Yet at the highway overpass — located to the west of the center — fighters were snapping selfies and shouting Shiite slogans. From a nearby pick-up truck blasting music and bearing a Hashed flag, cold water and yoghurt drinks were distributed to a group of soldiers.

"The country of Imam Ali forever! The country of Imam Ali forever!" they shouted, a popular chant in support of the revered Shiite holy man.

One Fallujah resident, Sheikh Hadi Muhamad Abdullah, who had returned home for the first time in two years, said he was shocked to see militiamen and Shiite graffiti in the city center, describing them as a personal insult.

"It's not a good sign," he said, arguing that the Shiite presence demonstrates that the government isn't serious about reconciliation with Iraq's Sunnis.

The government in Baghdad "believes that Fallujah is the center of terrorism in Iraq," Abdullah said. "But for us it's the center of resistance. The resistance started as pure, but others like Daesh corrupted it," he said, using the Arabic acronym for IS.

Although the military campaign in Fallujah is largely over, the special forces commander al-Saadi conceded that what comes next in the city could be equally important in containing IS.

"No kind of military solution alone will ever succeed in ending terrorism in Iraq," al-Saadi said. "You have to fight their mentality, the entire system."

Maj. Ali Hanoon, one of al-Saadi's deputies, fought in Fallujah alongside American troops in the mid-2000s. He remembers the day the contractors were hanged, and the brutal crackdown that followed.

He said that a decade ago, U.S. forces discovered that that the greater the hardship inflicted on the community, the more local support for militants grew.

He wasn't surprised when Fallujah fell to IS in 2014, he said, and he won't be surprised if the militants return.

"We'll be back again," Hanoon said. "Daesh will return, just under a new name and stronger."

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Associated Press writers Qassim Abdul-Zahra, Sinan Salaheddin and Muhanad al-Saleh in Baghdad contributed to this report.