WILLIAM BRANGHAM: With the ouster of former President Bashar al-Assad in Syria, the scale of his regime's mass killings and executions are coming to light more and more each day.

Yesterday, the United Nations said the new Syrian government was receptive to receiving help gathering evidence and prosecuting individuals responsible for war crimes.

Special correspondent Simona Foltyn visited some of the mass graves that have been discovered across the country and has this report.

And a warning: Some of the images and descriptions here are disturbing.

SIMONA FOLTYN: This is Tadamon, what used to be a bustling suburb of Southern Damascus.

After almost 14 years of war, it's a desolate wasteland with more bodies buried underground than people still living above it.

Human remains are scattered all around.

An air of dread and sanctity hangs over these grounds, where countless souls perished in summary executions.

Tamer Farrah lives just the street away.

He bore witness to some of the horrors that happened here.

TAMER FARRAH, Resident of Tadamon (through translator): This was the front line in the fight for Damascus.

All the killing and massacres happened here.

SIMONA FOLTYN: When the uprising began, Tadamon became a rebel stronghold.

Regime forces took control in 2012 and turned it into a killing field.

People picked up at checkpoints across the city were taken here to be executed in cold blood,their bodies burned and cast into unmarked graves.

TAMER FARRAH (through translator): We know that the football field was turned into a burn pit, where they burned bodies.

You could tell by the smoke and the smell coming out of it.

SIMONA FOLTYN: With time and rain, the bodies resurfaced from their shallow graves, like evidence of unsolved crimes returning to haunt the living.

Even the regime was bothered by the sight of its own atrocities.

TAMER FARRAH (through translator): There were a lot of bones here.

They took some away and then they agreed with the head of the municipality to blow up the buildings, so that the rubble would cover the rest of the remains.

SIMONA FOLTYN: We don't know how many people were killed here.

What we do know is that they were buried here in the middle of this residential neighborhood, much of which was destroyed during the war.

Now, some people still live here amid the rubble and the bones, but, during our visit, we didn't see any government employees or security forces guarding this site, which is essentially a crime scene.

Tadamon is known for one particular massacre.

It was committed in 2013 and captured on camera by the very people who perpetrated it.

The video was leaked a couple of years ago.

It shows the executioners as they marched dozens of men towards a large ditch.

One by one, the victims were cast into the grave and shot at close range.

This is the street where the massacre took place.

There, we found a family praying over a patch of dirt.

Ramez and his sister, Mahasen, only just found out that their brother was among those murdered here.

They watched a video over and over to find the precise location of the grave.

RAMEZ AZZAT SHARANDER, Brother of Victim (through translator): They brought them here, they kicked them and then they shot them.

SIMONA FOLTYN: Until yesterday, they had only seen a blurred version of the execution video, after years of uncertainty, finally the truth.

Mahasen is sure that the man in the red shirt is her beloved brother, Ahmed.

MAHASEN AZZAT SHARANDER, Sister of Victim (through translator): We saw him on YouTube we knew immediately that it was him.

SIMONA FOLTYN: Ahmed Azzat Sharander, pictured here on the left shortly before his killing, was 22 years old when he disappeared.

RAMEZ AZZAT SHARANDER (through translator): He had gone out to get bread, him and our neighbor's son.

And then he disappeared.

We were looking for him in the hospitals, at the police station, but we couldn't find him.

We were looking for him discreetly.

We were afraid they'd take us too.

SIMONA FOLTYN: That fear is now gone.

The family are demanding the immediate opening of the grave.

RAMEZ AZZAT SHARANDER (through translator): Even if all that's left are bones, I want to bury him with my hands in our grave, not for him to remain here, where everyone who passes steps on him.

SIMONA FOLTYN: The killing fields of Tadamon are not singular in scale or brutality.

Syria is littered with mass graves.

The new government has yet to put forth a plan on how and when these graves will be exhumed.

The lack of clarity has pushed some relatives to take matters into their own hands, according to Ammar Al-Salmo from The White Helmets.

AMMAR AL-SALMO, The White Helmets: People started to dig in the mass grave, and that could destroy the evidence.

SIMONA FOLTYN: The White Helmets, a civil defense organization, have recovered some human remains found above ground, but they and the country lack the expertise and technical capabilities to open mass graves.

The forensic experts who worked for the previous government are seen as complicit in the murders.

AMMAR AL-SALMO: There is no trust, because they issued a death certificate, normal death for those who died under torture and died under violence.

We need like a committee from international organization, from local organization, even from the government to -- like, to supervise the opening of that mass graves for the -- not only for taking them in expert way, also for the future, for the justice and for accountability.

SIMONA FOLTYN: We visit another, much larger mass grave not far from the notorious Sednaya prison.

Many of those murdered in Sednaya's torture chambers are presumed to be buried in this vast plot, some only days before the regime fell.

Unlike the informal mass graves we saw in Tadamon, this site was meticulously planned and built, a cog in the killing machine that was Assad's state.

Each of these cinder blocks you see on the ground marks the beginning of a new concrete vault that is around a yard wide, two yards deep, and maybe 20 yards long.

It's estimated that dozens, maybe hundreds of bodies were placed in each one of these vaults.

Now, we have counted 12 cinder blocks in just this section over here and there are five more sections nearby.

So you can do the math and estimate how many people might have been buried here.

The number is likely in the thousands.

We drove onwards to the garrison town of Kutefa (ph).

There, we met Sheikh Abdelqadar, a religious leader who witnessed the mass burial of victims of torture in 2013.

At first, the government disposed of the bodies in the town's cemetery.

SHEIKH ABDELQADAR SHEIKA, Religious Leader (through translator): They buried them here, around 40 to 60 people.

The grave was around four meters deep.

I oversaw their burial.

There was a patrol from military security.

They forbade anyone to enter or to watch from the roofs or film.

SIMONA FOLTYN: Sheikh Abdelqadar granted the victims their final burial rites.

SHEIKH ABDELQADAR SHEIKA (through translator): After the burial, I prayed for them.

One of the security officers told me: "Why do you pray for them?

They are terrorists."

And I told him: "These are Muslims from our country and I'm praying for them."

SIMONA FOLTYN: It all happened in plain sight of the town's inhabitants, but nobody spoke out.

SHEIKH ABDELQADAR SHEIKA (through translator): They would yell at the people so they don't approach.

They would forbid them to look.

If they saw someone on the roof, they'd tell them to get down.

SIMONA FOLTYN: As the government ramped up executions to an industrial scale, there was a need for a much larger burial ground.

The municipality assigned an empty plot just outside town with more space and fewer witnesses.

SHEIKH ABDELQADAR SHEIKA (through translator): The bodies were wrapped in a white sheet and there were numbers written on them.

SIMONA FOLTYN: The denial of individual burials was a deliberate act to torture even in death, while instilling terror among the living.

SHEIKH ABDELQADAR SHEIKA (through translator): When the prison sends them for burial, they don't put names.

Those who hand over the bodies hand them over without names and those who receive them also receive them without names.

SIMONA FOLTYN: Sheikh Abdelqadar believes these sites are holy and should not be disturbed through exhumation, but for many relatives of the missing, the unearthing of their loved ones can't begin soon enough.

For the "PBS News Hour," I'm Simona Foltyn in Damascus, Syria.