Theoretically, I believe (in addition to the police's duty to get the right guy) the accused is supposed to have a defence attorney who goes looking for exculpatory evidence.

AIUI America has an "adversarial" legal system, where the court is not involved in investigating the facts of the case. Instead, the defence and prosecution each make the strongest possible case and the judge and jury have to make their decision based on the evidence presented to them. For example the judge and jury don't get to question witnesses directly.

In contrast, Germany has an "inquisitorial" justice system, where the judges are actively involved in investigating the facts of the case and get to question witnesses and suchlike.