A mother and son have been fatally shot dead by her long-term partner in a double-murder suicide in Massachusetts, police have revealed.

The three victims were found  in a home in Main Street, Wilbraham, a suburb of Springfield, which has a population of just under 15,000 people.

Wilbraham Police Chief Edward Lennon told MassLive officers were called at around 9.30am on Tuesday, after a 'person' with access to the house made the grim discovery of three bodies inside.

Police confirmed all three had suffered fatal gunshot wounds, while the woman's partner's wounds were self-inflicted.

A dozen police cars were seen outside the light-grey home as detectives begin their investigation.

Hampden District Attorney Anthony D. Gulluni has not identified the victims, but they have been named locally as Sima Setian Pariseau, 56; her son, Dylan Pariseau, 27; and Pariseau's longtime partner, Peter Donaldson, 56, a senior sales account executive.

Information on when the shootings occurred has not been revealed.

In a statement, Gulluni said: 'We're going to let the medical examiner determine that.'