> The mercenary might get caught afterwards, before, or just be a Russian double agent from the start.
If you're going to make things up, it might be an entirely successful poisoning done by the girlfriend of the head of the gang, days after her mom's bills for cancer treatment were entirely paid off. Then, a rumor goes around that she killed him to get to a secret account, and the evidence is that her mom's bills were paid off mysteriously. Days later, she's found dead in an alley; people assume that she was a victim of another member of the gang - even the other members of the gang assume it, although no one is talking. Meanwhile, it was an agent that had been placed as a clerk at a local tax office, on his lunch break.
If you're going to make things up, it might be an entirely successful poisoning done by the girlfriend of the head of the gang, days after her mom's bills for cancer treatment were entirely paid off. Then, a rumor goes around that she killed him to get to a secret account, and the evidence is that her mom's bills were paid off mysteriously. Days later, she's found dead in an alley; people assume that she was a victim of another member of the gang - even the other members of the gang assume it, although no one is talking. Meanwhile, it was an agent that had been placed as a clerk at a local tax office, on his lunch break.
If we're making things up.