The FBI says it’s vetting all 25,000 National Guard troops deploying to Washington, D.C., for the inauguration, over fears of an insider attack by soldiers sympathetic to President Trump and the insurrection he incited on January 6. The FBI is also warning armed insurrectionists might pose in National Guard uniforms in order to launch attacks.

The warnings came amid an unprecedented security lockdown in Washington, D.C. The Coast Guard has closed the Potomac and Anacostia rivers, and heavily armed soldiers are patrolling a hardened security perimeter around the Capitol. The National Mall will be closed to the public during Wednesday’s inauguration — in place of the usual crowds, officials have placed nearly 200,000 American flags on the Mall.