The US justice department has given federal prosecutors the go-ahead to investigate alleged vote irregularities in the presidential election.
Such investigations are normally the remit of individual states, but Attorney General William Barr said this was not a hard and fast rule.
A senior official in his department resigned after reading Mr Barr’s memo.
President Donald Trump refuses to admit losing the vote, making unsubstantiated claims about voter fraud.
Meanwhile his campaign is seeking an emergency injunction in Pennsylvania to prevent President-elect Joe Biden’s victory being certified in the state.