Because of the semi-anonymous nature of Bitcoin, its used in ransomware attacks. Hackers breach networks and hold them hostage until payment is made to them. Hospitals and schools have been victims of these types of attacks. However, unlike cash, which was favored by thieves in the past, Bitcoin always leaves a trail in the blockchain that investigators can follow.
It is possible to track transactions, but it is impossible to know the personalities who participated in it. Attackers can claim or extort the keys and only users whose identity they know.