The FBI described it as an “active shooter situation” and said its Baltimore field office was assisting.
The Harford County Sheriff’s Office tweeted that the shooting involved “multiple victims” and warned that the situation was still fluid and asked people to avoid the area.
In a tweet, Maryland Gov. Larry Hogan said his office is monitoring the situation in Aberdeen and that the state stands ready to offer any support.
Associated Press writer Eric Tucker in Washington contributed to this report.