Four people were killed, including a toddler, and one seriously injured in a shooting spree in the U.S. state of Massachusetts early Tuesday.

The massacre happened at around 1:00 a.m Tuesday in Boston's Mattapan section. The four killed were found outside a house at 40 Woolson St., including a woman, a child in the woman's arms who was 2 to 3 years old, and two men. Another man suffered life-threatening injuries and was taken to hospital, police said.

"There is a significant amount of evidence located at the scene and we are involved in a very intensive investigation into the early morning hours," Police Commissioner Edward F. Davis told a news conference.

He said investigators thought the victims knew the shooter or shooters and called for the public's help in solving the case. Names of the victims have not yet been released.

Police said investigators were searching for a silver or gray Ford Explorer that might be connected to the shooting.