“This is the room where my whole family was killed,” says Safa Younes.

Bullet holes pepper the front door to the house in the Iraqi town of Haditha, where she grew up. Inside the back bedroom, a colourful bedspread covers the bed where her family was shot.

This is where she hid with her five siblings, mum and aunt when US marines stormed into their home and opened fire, killing everyone apart from Safa, on 19 November 2005. Her dad was also shot dead when he opened the front door.

Now, 20 years on, a BBC Eye investigation has uncovered evidence that implicates two marines, who were never brought to trial, in the killing of Safa’s family, according to a forensic expert.

The evidence – mainly statements and testimony given in the aftermath of the killings – raises doubts about the American investigation into what happened that day, and poses significant questions over how US armed forces are held to account.

The killing of Safa’s family was part of what became known as the Haditha massacre, when US marines killed 24 Iraqi civilians, including four women and six children. They entered three homes killing nearly everyone inside, as well as a driver and four students in a car, who were on their way to college.

The incident triggered the longest US war crimes investigation of the Iraq war, but no-one was convicted of the killings.

The marines said they were responding to gunfire after a roadside bomb went off, killing one of their squad members, and injuring two others.

But Safa, who was 13 at the time, tells the World Service: “We hadn’t been accused of anything. We didn’t even have any weapons in the house.”

She survived by pretending to be dead among the small bodies of her sisters and brother – the youngest was three years old. “I was the only survivor out of my entire family,” she says.

Four marines were initially charged with murder, but they gave conflicting accounts of the events, and over time US military prosecutors dropped charges against three of them, granting them immunity from further legal action.

That left squad leader Staff Sergeant Frank Wuterich as the only one to face trial in 2012.

In a video recording of a pre-trial hearing, which has never been broadcast before, the most junior member of the squad, Lance Corporal Humberto Mendoza is questioned and re-enacts events at Safa’s house.

Mendoza – who was a private at the time and was never charged – admits to killing Safa’s father when he opened the front door to the marines.

“Did you see his hands?” a lawyer asks him. “Yes sir,” Mendoza responds, and goes on to confirm that Safa’s father was not armed. “But you shot him anyways?” the lawyer asks. “Yes sir,” Mendoza says.

Outside the UK, watch the film on YouTube. Viewers in the UK can see more on Global Eye at 19:00 on BBC Two

In his official statements, Mendoza had initially claimed that after entering the house, he opened the door to the bedroom where Safa and her family were, but when he saw there were only women and children inside he did not go in, and instead shut the door.

However, in a newly discovered audio recording from Wuterich’s trial, Mendoza gives a different account. He says that he walked about 8ft (2.4m) into the bedroom.

This is hugely significant, according to forensic expert Michael Maloney. The Naval Criminal Investigative Service sent him to Haditha in 2006 to investigate the killings and he examined the bedroom where Safa’s family was shot.

Using the crime scene photos taken by the Marine Corps at the time of the killings, he concluded that two marines had entered the room and shot the women and children.

When we played him the recording of Mendoza saying he had walked into the room, Maloney said: “This is just amazing to me, what we’re listening to, and I’ve never heard this before today.”

He said it showed Mendoza was placing himself in the position where Maloney concluded the first shooter stood, at the foot of the bed.

“If you were to ask me: ‘Is this a confession of sorts?’ What I’d say is: ‘Mendoza confessed to everything except for pulling the trigger.'”

Safa had given a video deposition to military prosecutors in 2006 but it was never shown in court. In it, she described how the marine who opened the bedroom door threw in a grenade, which failed to explode, and then the same man came into the room and shot her family. Mendoza is the only marine who ever said he opened the door.

Another marine, Lance Corporal Stephen Tatum did not deny he took part in the shootings, but said he had followed the squad leader, Wuterich, into the bedroom and initially claimed he did not know there were women and children there because of poor visibility.

But in three later statements obtained by the BBC, Tatum gave a different account.

“I saw that children were in the room kneeling down. I don’t remember the exact number but only that it was a lot. I am trained to shoot two shots to the chest and two shots to the head and I followed my training,” Tatum told the Naval Criminal Investigative Service in April 2006.

A month later, he said he “was able to positively identify the people in the room as women and children before shooting them”.

And then a week after that, he said: “This is where I saw the kid I shot. Knowing it was a kid, I still shot him.” He described the child as wearing a white T-shirt, standing on the bed, and having short hair.

Tatum’s defence lawyers claimed these later statements had been obtained under duress. Charges against Tatum were dropped in March 2008, and the statements were disregarded at Wuterich’s trial.

Forensics expert Michael Maloney said the statements by Mendoza and Tatum point to them being the two marines who shot Safa’s family. He believes Mendoza went into the bedroom first and Tatum followed “firing across the head of the bed”.

We put the allegations to Mendoza and Tatum. Mendoza did not respond. He has previously admitted to shooting Safa’s father, but said he was following orders. He was never charged with a criminal offence.

Through his lawyer, Tatum said he wants to put Haditha behind him. He has never withdrawn his testimony that he was one of the shooters in Safa’s house.

Maloney told the BBC that the prosecution “wanted Wuterich to be that primary shooter”. But before Maloney was able to testify, Wuterich’s trial ended in a plea deal.

Wuterich maintained he could not remember what had happened in Safa’s house, and agreed to plead guilty to one count of negligent dereliction of duty – a charge unrelated to any direct involvement in the killings.

Wuterich’s military lawyer, Haytham Faraj, a former marine himself, said the punishment was “tantamount to a slap on the wrist… like a speeding ticket”.

Neal Puckett, the lead defence lawyer for Wuterich, said the whole investigation and prosecution against his client was “botched”.

“The prosecution, in granting immunity to all their witnesses and dismissing all their charges… essentially rendered themselves incapable of achieving justice in this case,” he said.

Haytham Faraj agreed the process was deeply flawed.

“The government paid people to come in and lie, and the payment was immunity, and that’s how they misused the legal process,” he told the BBC.

“The trial of Haditha was never meant to give voice to the victims,” he added.

He said that survivors’ “impressions of a show trial with no real outcome, with no-one being punished, was right”.

The US Marine Corps told us it is committed to fair and open proceedings under the Uniform Code of Military Justice, ensuring due process of law. It added that it would not reopen the investigation unless a wealth of new, unexamined, and admissible evidence was introduced.

The lead prosecutor in the case did not respond to the BBC’s request for comment.

Now aged 33, Safa still lives in Haditha and has three children of her own. She says she can’t understand how no marine was punished for her family’s deaths.

When we show her the video of Mendoza, she says he “should have been imprisoned from the moment the incident happened, it should have been impossible for him to see the light of day”.

“It’s as if it happened last year. I still think about it,” she says of the day her family was killed.

“I want those who did this to be held accountable and to be punished by the law. It’s been almost 20 years without them being tried. That’s the real crime.”

Additional reporting by Namak Khoshnaw and Michael Epstein