"This is basically one of the most serious and significant crimes we have had in this city," said police Sergeant Deanna Nollette.
"The number of young people who witnessed what happened is disturbing."
Neighbours had told police that the rented home where the shooting took place was known as an all-night party spot.
The gunman had left the party early on Saturday morning, returned with weapons, including a pistol and a shotgun, and opened fire.
The shooting occurred in the fashionable Capitol Hill neighborhood.