The police department in the city, the second-largest in New England after Boston, was not immediately available for comment.
Worcester Mayor Joseph Petty, the city's Acting Manager Eric Batista and Worcester Police Chief Steven Sargent pledged to cooperate with the investigation, the justice department said.
Since then, the justice department has opened dozens of similar investigations across the nation.
The justice department launched a so-called "pattern or practice" investigation in Minneapolis a day after a jury found the officer, Derek Chauvin, guilty of murder.
In 2018, the Justice Department opened an investigation of the Springfield Police Department.