In her Long Island home, Cat Colvin sorts through the family photo album.

She cared for the victims, not herself I once met Marie Colvin when we spent a few days together on a chemical warfare course run by the British Army at Salisbury, south of London. She was already famous as a war correspondent, although she didn't like that description. Marie was about people. Telling the world about the suffering of the innocents … like the families struggling to survive the relentless attack on Homs by the regime of Bashar al-Assad. She didn't appear interested in the mechanics of war. But she cared deeply about the victims and wanted us to care too. And do something about it. Marie Colvin knew about the suffering first hand. When we met she'd only recently returned to the craft she so loved after losing an eye in a grenade attack in Sri Lanka. Perhaps rather insensitively I asked her how she felt about returning to journalism after the pain and fear of that horrendous experience. She wouldn't engage. The lady with the eyepatch seemed uninterested in talking about her struggles. It was all about others, powerless people stuck in places they couldn't escape with no magic passport to whisk them to safety. Yes, she had suffered terribly. That was obvious. But she had a way out and most of those she wrote about could not escape. And without Marie Colvin's courage and determination, the many crimes against humanity would have gone unrecorded. The suffering of those killed and maimed by barrel bombs, chemical attacks and state sponsored starvation would have vanished, as if it didn't happen and didn't matter. She made it matter. And that, according to her family is why she was targeted … murdered to stop her reports that humanised those the regime demonised; gave voice to those without power, without hope - Philip Williams

"That's dad and mum and Marie as a baby," she says thumbing through the old photos.

There is a lovely picture of her with her sister, the famous war correspondent, Marie Colvin. You can feel the warmth, the closeness — and the pain.

"It gutted me, even when I talk about it I still get emotional about it. I really didn't believe it at first," she says, recalling the awful moment she was told her beloved big sister had been killed by shells in the besieged Syrian city of Homs in 2012.

From the beginning, Ms Colvin suspected her sister's death was not simply a case of being in the wrong place at the wrong time.

Now, after years of forensic investigation, she is certain Syrian President Bashar al-Assad and his cronies deliberately targeted the makeshift media centre where Marie Colvin and others were sheltering from the intense bombardment.

In a six-year search for evidence, the Centre for Justice and Accountability has produced eyewitness accounts of the killing of Marie, a French photographer and the wounding of other foreign journalists.

"It's incredible: the calculation; the planning; the organisation of the military; and putting the full force of the Syrian military toward killing journalists," Ms Colvin says.

"It was really shocking."

What angers Ms Colvin the most are the testimonies about how the Syrian President's brother and his close associates reacted to news they had murdered the woman whose reports had so angered the regime.

"The most disturbing part of the evidence to me was eyewitness testimony of the celebrations that happened after Marie's death, calling her 'American dog' and 'blind bitch', and cars being rewarded to those who had successfully killed Marie," she says.

"The thought of them laughing and having a party celebrating her death is just infuriating."

Veteran correspondent: Marie Colvin (second left) sits with Libyan rebels in Misrata on June 4, 2011. ( Reuters: Zohra Bensemra )

Launching civil legal action in a Washington court, it's possible Syrian assets frozen by the US government could be used to compensate Ms Colvin's family for their suffering over the past six years.

Ultimately Ms Colvin wants Mr Assad held accountable.

"If I could throw him in jail right now, bring criminal charges myself, I would love to do that," she says.

"The tools available to me are in the civil court, which is monetary damages. The way I look at that, every dollar I squeeze out of him is one he can't use to buy chemical weapons and barrel bombs."

The civil legal action would never have been launched without six years of investigation by lawyer Scott Gilmore from the Centre for Justice and Accountability (CJA).

He spent months talking to a former Syrian intelligence officer code-named "Ulysses", who described in great detail the Syrian Government's hunt for foreign journalists.

"There is no international tribunal with jurisdiction over these war crimes in Syria," he says.

"And so for myself, for CJA and for the Colvin family, this is really an opportunity for us to provide hard evidence of the meticulous planning, the cold-blooded killing of civilians by the regime, as a first step to show what is possible on behalf of thousands of Syrian victims."

Court documents reveal celebrations that followed Marie Colvin's death. ( ABC News )

The documents just released by the US federal court are perhaps the most comprehensive dossier implicating the Syrian Government in multiple war crimes.

Mr Gilmore hopes the Colvin case is the start of a process that will lead to a day of reckoning for Syrian war criminals.

"I have no doubt that some day they will be in the dock facing prosecution. It might take a long time, I mean the path of, you know, international justice can take decades unfortunately, but it might be a matter of making sure that there is no safe haven for them," he says.

With Mr Assad currently buoyed by the Russian and Iranian military, that day may come, but not any time soon.