District of Columbia police said shots were fired into a group of people in one of the District's deadliest shootings in years, The Washington Post reported.
Officials said three of the victims died Tuesday and the fourth Wednesday. Five other people were injured.
Police said a 10th person was found shot a few blocks from the scene but they said they weren't sure whether he was part of the gunfire exchange.
Three people were in custody but charges hadn't been filed, District of Columbia Police Chief Cathy Lanier said.
Two police vehicles crashed during a pursuit of the three individuals and four police officers suffered minor injuries, officials said.
Lanier said many people were outdoors in the District of Columbia's southern tip when a gunman began "spraying (bullets) into a crowd."
A man told the Post he heard sounds he described as "pat, pat, pat, pat, pat," followed by a loud boom.
Then, "all I saw was bodies dropping," the witness said. "It was like Vietnam."
Officials said a motive wasn't immediately known.
Sources told the Post they were investigating whether the shootings were tied to two incidents in March in the southern part of the district.
The number of people shot Tuesday may have been the most in one incident in Washington since 1994, when 10 people were wounded, one fatally, the Post said.