Addana, Syria (CNN) -- From the moment we cross the Turkish border into Syria, evidence of the Islamic State in Iraq and Syria's draconian and brutal rule lingers.
More than a month after the group was driven from the town of Addana, residents here drive through the streets, pointing out where ISIS fighters carried out executions and left bodies to rot for all to see.
"That's where they had one of their checkpoints," says rebel fighter Abu Sa'ed, pointing to a small concrete building on the side of the road as we speed past.
ISIS arrived in Addana about a year ago, initially welcomed in the conservative town by Islamist fighters. But within a few months, ISIS had entrenched itself and begun exerting its harsh order through what one fighter calls "terrorism and punishment."
"ISIS came in and took over one area and announced it was an Islamic state and did whatever they wanted," Abu Sa'ed says.
In the passenger's seat, fellow fighter Abu Jaafar sighs, his AK-47 trained out the window.
"They used to leave the bodies of people they executed at the checkpoint for days," he says. "The corpses would rot. No one could avoid looking at them."
Amid the civil war in Syria, another war is taking place -- one that pits moderate and Islamist rebels against radicals from ISIS, a group so radical that even al Qaeda has reportedly distanced itself from it. Both groups of fighters are opposed to the government of Syrian President Bashar al-Assad.
For months, reports have been emerging from northern Syria about ISIS atrocities.
In Addana, bullet holes riddle the walls at a former ISIS prison. Most of them are at the same level -- the average height of a man.
"Those are mostly from the executions," says Abu Jamal, another fighter. "Others are from the battle."
At the town's courthouse, a fresh coat of white paint covers the black mark left by ISIS -- known to paint buildings black as they take over Syrian towns. Executions would take place out front. In the back, freshly dug dirt marks where some of the bodies were buried in mass graves.
Abu Jamal points to a pile of burning trash. ISIS beheaded one of the top rebel commanders -- placing his head in that very spot -- in broad daylight when the market was packed with people as a warning: anyone who dared oppose them would have the same fate.
In early January, rebel groups banded together and launched an offensive against ISIS in Addana and other areas in northern Syria.
"We had to leave the fronts with the regime," Abu Jamal says, "and fall back to fight ISIS and liberate the already liberated areas."
Still, many in Addana are too afraid to speak openly. So deep is their fear, they don't even want to be seen with us.
Next to the former prison, a family waits, hoping for closure. Volunteers shovel dirt from a grave they dug up before. Video filmed at the time shows four contorted corpses. It's among many mass graves rebel fighters have unearthed.
"We found a foot, a shoe, and a jacket," Addana resident Ayoush Ali tells us.
Her neighbor Mohammed Ismail joins us. His two younger brothers are missing, he says.
"It's my brother's jacket. He just went out to get sugar and tomatoes."
A man drops a pile of children's socks, covered in dirt, on the ground.
"His wife wanted socks for their kids," Mohammed says, still disbelieving.
The day after we meet Mohammed, the bodies are dug up. It's three of his brothers who were killed by ISIS. He had thought one of them was in jail.
He crouches on the ground, covering his face, shoulders shaking as he sobs.