A police officer has been shot dead on the campus of the Massachusetts Institute of Technology (MIT) near Boston, according to officials.
The incident took place in Cambridge. The police officer was responding to an emergency call in the area when the gunman shot him.
The victim suffered "multiple gunshot wounds" and was taken to a nearby hospital where he was pronounced dead. The officer has not been immediately identified.
The campus has been cordoned off by the police while officials continue to pursue the gunman.
The university on its website warned people on the campus to "remain indoors" as the situation continues to remain "very fluid" and "extremely dangerous."
A statement from the Middlesex District Attorney's office and Cambridge police said there were no other victims in the incident which appears to be unrelated to the Boston marathon blasts.