More than 70 US Capitol Police officers have either resigned or retired in the wake of the deadly January 6 riots at the US Capitol, according to a police union boss.

Some 140 Capitol Police officers were injured when an angry mob of Donald Trump's supporters stormed the US Capitol building to stop the certification the election victory of Joe Biden.

Then on April 2, a delusional and suicidal knife-wielding attacker mowed down two Capitol cops, killing one and leaving the other critically injured.

'What keeps me awake at night is not the challenge of hiring and training more police officers, but keeping the officers we have right now,' said USCP Labor Committee Chairman Gus Papathanasiou in a statement to CNN. 'We have many officers on the fence about whether to stay with this department.'