Gunfire broke out at a park in Greenbelt, Maryland, where hundreds of high school students had gathered while skipping school on Friday, wounding five males aged 16 to 18, police said.
One victim was in critical condition and the others were in stable condition, Greenbelt Police Chief Richard Bowers told a press conference.
Police were searching for one suspect but there may be others, Bowers said. No motive had been established.
"There's absolutely no reason that this occurred. It is senseless, it is chronic in our society, and we have to do something to stop it," Bowers said. "It's maddening, honestly."
There have been 120 such mass shootings in the United States in the first 110 days of this year, when defined as an incident with four or more people shot or killed, not including the shooter, according to the Gun Violence Archive.