Freddy Mukuza’s final moments were witnessed by a friend, who stood helpless, 50m (160 ft) away.

When he heard that Freddy had been shot – by M23 rebels he was told – he and others rushed to the scene in Goma, in eastern Democratic Republic of Congo.

“When we arrived, we found Freddy still breathing, and wanted to take him away, but the M23 did not allow us,” says the friend, who we are calling Justin.

“When we insisted, they fired bullets into the ground as if to say: ‘If you dare cross this perimeter, we will kill you as well.'”

So they had to keep their distance, as Freddy, 31, took his last breath. Only then did the M23 allow them to approach and take away his body.

Shortly before the killing, three pick-up trucks full of rebel fighters had come to Freddy’s neighbourhood – Kasika.

It was around 15:00 on Saturday 22 February – almost a month after the rebel group had captured Goma in a rapid advance through the east of the country.

Within an hour or so, between 17 and 22 people had been killed, mostly young men, according to our sources.

We have gathered detailed accounts from residents, who cannot be identified, for their own protection.

We asked the M23 for a response to the allegation that they carried out a mass killing in the neighbourhood. They did not reply.

Officials in Kasika have not released a death toll, and there is little or no prospect of an independent criminal investigation into what residents are calling a massacre.

But locals insist the M23 is the only armed group which can operate freely, and shoot to kill in broad daylight in Goma.

Since taking the city at the end of January, the rebels have been in complete control. During the 18 days we spent on the ground, their authority was absolute.

They have been accused in the past of carrying out atrocities in other areas.

The heavily armed rebels do not act alone. They are backed by neighbouring Rwanda, according to the UN and the US. Rwanda denies this, though it no longer denies having its own troops in DR Congo, saying they are there in self-defence.

It is believed the M23 targeted Kasika because of a former Congolese army base in the area.

The Katindo camp is now closed but some of the soldiers and their families remain in the district.

“Not all the soldiers were able to run away,” a local resident explains. “Some threw away their guns and stayed about the neighbourhood.”

But Freddy Mukuza was a civilian – a married father of two, struggling to get by. When hard times came, he earned a living by taking passengers on his motorbike.

He was also an activist and songwriter who rapped about the many problems in his homeland – a country rich in minerals whose people are amongst the poorest in the world.

DR Congo is known as a place of corruption and instability – and of conflict, stretching back 30 years. That is if the country and its suffering are remembered at all.

Sexual violence is endemic. The government is weak, at best.

There was plenty for Freddy to rap about.

One of his songs is called Au Secours (Help in French), the lyrics full of questions that have gone unanswered:

“Who will come to the aid of these people? Who will come to the aid of these raped women? Who will come to the aid of these unemployed men?… The people are in danger, they don’t have enough to eat. They [the authorities] buy jeeps.”

On the day of his death, Freddy was moving to a new rented home in Kasika. His brother-in-law was helping him put a tarpaulin over the roof.

His sister-in-law was there too, getting the house ready for Freddy’s family. When they heard the shooting, they were inside and rushed to shut the door, but they were seen by the M23.

The rebels shot and killed Freddy’s two in-laws, according to his friend Justin.

Since then, Justin has barely left home, not even to earn money. His family is surviving on vegetables and fruit. Tea is now a luxury they cannot afford.

He has stopped his children going to school, for fear they might be taken from their classrooms by the M23 and forcibly recruited.

“We believe it is more important that they stay alive,” he says.

His world has shrunk to his own four walls. There is the constant nagging fear that the rebels could return hunting for young men.

Just the sight of one of their pick-up trucks in the street sends locals running, he says.

These days it is rare to find a group of young people talking together, he tells us, and neighbours no long share gripes about the authorities as they did before the rebel takeover.

“Before, there was bad governance, but we were free,” he says. “There was embezzlement. There was mismanagement and we spoke out about that. We had the chance to go to court. Today, there is bad governance, but we live in terror and silence.”

Justin is speaking to us because he wants Freddy Mukuza to be remembered, and he wants the outside world to know about life and death under the M23.

Since the killings, Kasika has been shrouded in fear. Local journalists have not reported the story.

But a shaky video was posted on social media the next day, 23 February, which appears to show some of the victims: 10 bodies are visible – dumped in a tangled heap, in an unfinished building. It is unclear if any of the dead were soldiers.

None are in uniform and there is no sign of any weapons.

In the background there are screams and shouts. One woman repeats over and over: “There are 10 of them,” as she moves from body to body.

“They are going to finish us all,” she says. “They killed all these young people. Isn’t that Junior? I think it is him. He is a house builder.”

Without the video, news of the killings might not have spread beyond the neighbourhood.

But the footage had the power to shock, even by the violent standards of DR Congo.

Our sources say it is authentic. One confirmed that the location shown is in Kasika.

He visited the place after the bodies were moved. And he recognised one of those seen crying in the video, from around the neighbourhood.

Two of our sources say the youngest to die in Kasika was a boy aged 13-14. The teenager was inside his own home, hiding behind his sisters.

“The M23 said: ‘If this boy does not come with us, we will kill all of you,'” one man told us.

The boy was then led away to his death.

There was also a young woman among the victims. She had been selling milk on the overcrowded streets.

Also killed – another street vendor, in his twenties.

When the shooting started, he was sitting in his usual spot – on the pavement outside his own front door, selling airtime for mobile phones and home-made doughnuts.

He was overheard pleading with the rebels: “I’m not a soldier.

“I just sell airtime. Look, these are my things – my airtime and my basket of doughnuts.”

Then he ran. One of his friends takes up the story. We are calling him John.

“I was in the house, and I heard gunfire,” John tells us. “People were saying: ‘They are taking young people by force.’ I saw people running, including my friend, so I ran with them.

“When we reached the main road, there was shooting, and I heard gunfire behind me and somebody fell.”

That was the doughnut seller.

Despite his age, he was still in secondary school, in his final year. He was a keen student who had a late start because his family could not afford to educate him.

But John says: “Like all young people, he had a dream.” In his case, it was to be an engineer.

John says the rebels did not care who they killed.

“There was no inquiry before shooting,” he tells us. “They just shot at everyone who was present, and at people who ran away, in two different directions.”

When the M23 captured Goma, they announced they had no prisons. John says no further explanation was needed: “That meant whoever is presumed to be a government soldier, or a thief, or whoever makes a mistake, will be killed – immediately.”

Weeks on, few have dared to speak out. “No-one wants to be next,” John says.

Bereaved families have held small hasty burials – without the usual mourning at home.

“The rebels didn’t want any funerals,” says one resident, who we are calling Deborah. “They didn’t even want people to cry. We thought they were coming to bring peace, but instead they came to exterminate us. They took everyone they found on the street.”

As the men were being rounded up, she tried to step outside. The rebels ordered her back in, at gunpoint.

Denis Baeni was on his way home when the rebels arrived in Kasika. He dashed into a small shop to hide with a few others, our sources say.

The primary school teacher got his ID card out of a pocket. He may have thought that would save him, by proving he was a civilian.

A neighbour – with knowledge of the events – tells us what happened. We are calling her Rebecca.

“They heard a voice from outside asking: ‘Are there any soldiers?'” Rebecca says. “They said no but the M23 took them out of the shop.”

The men were told to walk a short distance to a half-built house where they were “assembled for execution”.

“There was so much gunfire,” she says. “It was so close. There were 21 people killed all at once from our neighbourhood. Many were just passing by.”

Rebecca insists they were all civilians. “Not one was a soldier,” she says.

Denis leaves behind two children, who he was raising alone.

Death is not the only danger here. Locals also face the risk of being recruited to fight – willingly or otherwise.

“Nowadays men have to be home by 17:30,” says Rebecca. “By 18:00 it’s dark, and they can take you very easily.”

As families in Kasika are forced to stifle their grief, the M23 are continuing their sweep through eastern DR Congo.

After Goma, they took control the city of Bukavu in mid-February. They have threatened to go all the way to the capital, Kinshasa, 1,600km (almost 1,000 miles) away.

They claim they are revolutionaries battling a failed state, and defending the rights of minority Tutsis.

Human rights groups paint a very different picture.

They have accused the armed group of a catalogue of abuses since its foundation in 2012 – including systematic shelling of civilian areas, gang rape and “summary executions”. The allegations have been documented in a series of reports.

In a recent BBC interview, I asked rebel leader, Corneille Nangaa, for a response. He heads a coalition of political parties and militias – called the Congo River Alliance – which includes the M23.

“I didn’t see the reports,” he said. “I cannot respond for the report that I didn’t read”. He also said he was not worried by the allegations.

Pushed on why he had not read the reports, he said: “Give me one, I will be reading it.”

Nangaa, a former head of DR Congo’s electoral commission, alternates between combat fatigues and smart suits.

He is presented as the unarmed and unthreatening face of the rebels, but the Congolese government is offering a $5m (£4m) reward for information leading to his arrest.

The rebels are not alone in having a history of brutality. The same applies to the Congolese army, and to many of the other armed groups in eastern DR Congo.

But the M23 are now the only authority in parts of the east, and millions of Congolese are at their mercy.

As we spoke to one resident of Kasika, his wife called him, asking him to come quickly to take their eight-year-old son from school.

Panic was spreading because of reports the M23 were taking children from their classrooms.

He got his child home safely but fears for the future.

“We are all traumatised. They said they came to liberate us,” he said. “But now it’s like they are taking us hostage. “

Additional reporting from the BBC’s Wietske Burema.