The US criminal (in)justice system has at least two major flaws. One is some police and prosecutors prioritizing convictions over justice, and the other is them seeking extremely harsh penalties for even minor crimes. Combine that with racial prejudice and the combination is deadly because you can end up with innocent people serving extremely long sentences and even being executed.
As an example of the first type, I wrote recently about how many police line-ups are conducted in a way that witnesses are subtly influenced by the police officers who have arrested a suspect to pick the suspect, instead of using a double-blind method.
As an even more extreme example of the two flaws together, Emily Bazelon writes about a case in 2012 in which 19-year old Yutico Briley was wrongfully convicted of a crime because the prosecutors were more interested in getting a conviction than justice.