Trump-Biden: Security fears cloud build-up to inauguration - BBC News
Thousands of National Guard troops are being deployed to bolster security in Washington DC.
In his letter to staff, circulated on social media, Chad Wolf said he had hoped to remain as acting secretary to homeland security until the end of the Trump administration.

"Unfortunately, this action is warranted by the recent events, including the ongoing and meritless court rulings regarding the validity of my authority as acting secretary," he said, "which serve to divert attention and resources away from the important work of the Department in this critical time of a transition of power".

Wolf's resignation comes after he last week called on Trump and all elected officials to "strongly condemn" the Capitol riot.

His exit throws the department into turmoil just as it is gearing up for inauguration of Joe Biden as president on 20 January, which has been designated a national security special event.