Two people were stabbed at a home outside Boston and then two at the nearby Silver City Galleria mall by the assailant, who had crashed a car into the front of a store, Massachusetts State Police said in a statement.
The suspect died after being shot by an off-duty law enforcement officer at the shopping center, state police said.
Taunton Fire Department dispatcher Mike Marshall said reports started coming in around 6:37 p.m. and that the department sent out "a lot" of ambulances.
The victims were taken to hospitals, some facing life-threatening injuries, state police said. Later, police announced that the assailant and two of his victims had died, and said more information would be released by a county prosecutor.
Witnesses told television station WHDH-TV that they saw people running out of the mall and heard gunfire. The station reported that people in the area, about 40 miles (64 km) south of Boston, saw police officers with guns drawn and that the mall had been evacuated and put on lockdown.
The motives for the attacks were unclear. A number of police units were at the crime scenes, authorities said.
A representative of the mall, which has 125 stores, a 12-unit food court, restaurants and a cinema, could not immediately be reached for comment.