STORY: Washington Metropolitan Police Chief Robert Contee did not immediately identify the suspect, but told reporters that police were no longer seeking the "person of interest" announced hours earlier.

That was 23-year-old Raymond Spencer of suburban Fairfax, Virginia, whom police identified from a video he had posted on social media that appeared to show gunshots fired from the vantage point of an upper-floor window, with the misspelled label: "Shool shooting!"

Contee said the video "looks very much to be authentic", but could not yet confirm if the footage was streamed live or a recording posted afterwards.

With help from the video and eyewitness reports, police managed to pinpoint the gunman's position to the fifth floor of an apartment building" and ultimately "breached the location where the suspect took his own life," Contee said.

Police seized more than half a dozen firearms, including several rifles, and large amounts of ammunition in the apartment, which had been arranged in a "sniper-type setup" with a tripod weapons mount.

"His intent was to kill and hurt members of our community," but investigators had yet to determine a motive, Contee said, adding that the shooter had fired at least 20 shots.

Three of the four victims required hospital treatment, including a 12-year-old girl who was hit in the arm, according to police. The fourth victim was treated on the scene for a slight graze wound.