In July 2018, a horrifying video began to spread online.

On a dusty path, somewhere in Sub-Saharan Africa, a group of gun-wielding men in camouflage fatigues

are frog-marching two terrified-looking women down the road, slapping them and pushing them as they go.

One woman tries to whisper to a young girl she is holding by the hand.

The other woman carries a baby strapped to her back with a shawl.

The video plays on.

The women and the young girl are forced to the floor and blindfolded.

The baby remains on the woman's back.

Then three men draw their weapons and shoot all four of them dead.

It's a sickening sight and one that's not easy to forget.

But where had the video come from?

Who were the women and who were their killers? Those were the questions Aliam Leroy wanted

to find out. I'm Maeve McClendigan. This is the tip-off. My name is Ayomi Roy. I'm an open-source

investigator. Aliam works for BBC Africa and has a very particular set of skills. He's something of

an expert in open source investigations, using information already published in maps, databases,

or other places online to get to the crux of a question. When he arrived in at work at the BBC offices one morning,

soon after the video of these awful killings was published, his colleagues had a challenge for him.

So normally I walk into the office, open my computer, look at Twitter. This first thing I would do

and see what's coming from Africa on the day. But for this one, I just got into the office.

And before I sat down down there's someone from

Focus on Africa which is a program at the BBC that came to me and say hey have you seen

this video coming out of or we don't know where but allegedly Cameroon and Karen you

...