I think this information here said FBI probably has the private key but for me, I can't say how they were able to get hold of the private key.
Well.. that is the most important question.
Why should the hackers send the coins to a wallet, that is controlled by FBI. There are all sort of possibilities in play here. My theory goes like this:
The FBI guys arrested an individual or a group of individuals, who were operating a Bitcoin mixer. The arrested individuals shared the details of all their cryptocurrency wallets with the FBI (including the private key). The hackers think that the mixer is still in operation and they send the stolen coins to the wallet, in order to wash them.
But someone in this forum claims that the wallet is linked to the Gemini exchange. In that case, I can't really explain what happened.