WJLA reports that the attack occurred close to U Street, not on a back alley, and there would have been plenty of witnesses. The Metro station is across U-Street. That makes this attack particularly shocking.
Update: April 20, 2018
NBCWashington has a story and video on a vigil at Dupont Circle in the evening of April 19 here. WJLA has carried the story of the gathering but did not put it yet on its website. No suspects have been apprehended yet (as of noon Friday). DC police say that hate crimes in the city have increased since the end of 2015, apparently as the political climate became more divisive with Trump's candidacy and social media bots.