Two members of the National Guard were shot and seriously wounded just blocks from the White House in Washington DC.
They remain in a critical condition and the suspect, an Afghan national, is in custody.
In response to the attack, President Donald Trump said he would deploy another 500 troops to the streets of the nation's capital.
Just after 14:00EST (19:00GMT) on Wednesday, two members of the National Guard, both from West Virginia, were shot at close range near Farragut Square in downtown Washington DC.
The soldiers had been on a high-visibility patrol near the corner of 17th and I streets, an area where many office workers mill around at lunchtime.
