Former US President Donald Trump in Washington, DC. (Documentary photo: AFP/VNA)

In an unprecedented indictment of a former US President, Trump pleaded to 37 federal charges, including unlawfully retaining government secrets and conspiring to obstruct justice. Trump says he is innocent and is being unfairly targeted by prosecutors because he's running for President again. 

This is the second time Trump has been indicted on criminal charges. In April, he had to appear in court in New York to answer charges of falsifying business documents. Trump is also being investigated for violations of election law in the state of Georgia.

Those two cases involve violations of state laws, but the classified documents case involves federal crimes, and is the first time in history that a former US president has been indicted by a federal court.