When we left off last week, Heidi and the team had just had a breakthrough.

Serving US spies had told them they were onto something big.

That there was intelligence linking 14 deaths that they had identified to either the Russian mafia or the Russian state. Fourteen cases of people who had died in awful circumstances, impaled on iron railings, stabbed all over their bodies, hanged in their own bathrooms.

There was something incredibly sinister going on.

The team had been working for months on the project, meticulously digging through data,

but in early 2017 they started to ramp up their reporting,

fanning out to interview scores of people. And it was then that strange things started to happen.

One night, after a long day in the office, Heidi was heading home to her London house,

when a parked car on her street caught her eye.

A man in a large black car appeared outside my house.

Nothing that unusual there.

And perhaps her nerves were just on edge from picking through the gory details of horrible cases.

But then the next night, heading home again, there he was.

And then again the next night.

And he was there every single night, almost all night, just for weeks on end.

And when one of my neighbours approached him after I'd said that this was freaking me out a bit,

he made some quite bizarre comments about how he was there to monitor someone,

and he was there just to watch the houses in case anything happened.

And so that felt potentially a little bit threatening.

But that wasn't all.

Other people in the team were noticing strange things too.

And we also had another member of the team return home to find that items had been moved

...