(SOUNDBITE) (English) VICTIM OF HUMAN TRAFFICKING, BLESSING OBUSON, SAYING: "When I was back in Nigeria in the hotel where we were staying, there were close to 23 girls there with fan ID.
And they were all coming to Russia but with different madams." Police raids like this one on an apartment in Moscow freed women tricked into sex slavery during Russia's World Cup last year.
All Nigerian, like Blessing Obuson, who traveled there on what was called a "Fan ID" - basically a temporary visa for ticket holders during the games.
The Fan IDs didn't allow the right to work but Obuson wanted to get a job as a store clerk and earn some money for her family - she'd left her two year old daughter behind.
Instead she found herself locked in an apartment, forced into sex work with 11 other women.
Police say one was killed by a man who refused to pay.
(SOUNDBITE) (English) VICTIM OF HUMAN TRAFFICKING, BLESSING OBUSON, SAYING: "During the period when the girl died my madam told us that it was not a client that killed her, that she ran away from her madam, that's how she found herself dead.
That really frightened us, that really made us not to leave the apartment, that made us not to tell anyone our story.
So these were the facts that really kept us back in the apartment." Reuters met eight women with similar stories.
Activist groups estimate 2,000 Nigerian women were brought in on fan IDs.
Obuson's unlikely savior was when she encountered a client that spoke English.
She told him her story and he got anti-slavery activists involved - like Kenny Kehindo.
(SOUNDBITE) (English) ANTI-TRAFFICKING ACTIVIST, KENNY KEHINDO, SAYING: "When they are here in Russia it is the same process.
They are taken the different houses, they cease their documents and they are forced to work as sex slaves.
And most of them were threatened that if they report to the police, they will have a problem." Neither Russian police nor the Nigerian embassy in Moscow replied to Reuters request for comment.