Pictured: 'Long term partner' of slain Massachusetts woman, who was killed alongside her adult son, before gunman took his own life

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. 

Sima Setian Pariseau, 56, (right) and her son, Dylan Pariseau, 27, were found fatally shot dead by her partner in a double-murder suicide in Wilbraham, Massachusetts

Sima Setian Pariseau, pictured with her long-term partner Peter Donaldson, 56, who is believed to have shot her and her son, Dylan, before turning the gun on himself

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 discovery 

The discovery was made at this home in Main Street, Wilbraham, a suburb of Springfield, which has a population of just under 15,000 people

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.' 

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While there is no threat to the general public, residents are being asked to avoid the area of Main Street between Highmoor Drive and Burleigh Road, NBC Boston reports.

Sima, who also has a daughter, worked as a medical coder for Baystate Medical Centre for more than 20 years and only recently started a new job at Mass General Brigham, specialising in neurology, according to her LinkedIn profile. 

She graduated in 1988 from the now-closed Becker College, a private college in Worcester and Leicester, Massachusetts.

Last April she became a volunteer advocate for foster children, advising young people in care and was listed as a trustee on the board of St Gregory Armenian Church in Indian Orchard, located a short drive from her home.

Her son, Dylan, was a senior medical research assistant researching into cardiovascular disease at Verve Therapeutics, headquartered in Boston, according to a profile on recruitment platform SignalHire.

Dylan, was a senior medical research assistant researching into cardiovascular disease at Verve Therapeutics, headquartered in Boston, according to an online profile

Sima was a medical coder, who recently became a volunteer advocate for foster children and was a trustee on the board of her local church, St Gregory Armenian Church, in Indian Orchard

Dylan went to his local Minnechaug Regional High School, in Wilbraham, before going on to study biology and chemistry at Keene State College.

He was also formerly a  research assistant at Harvard Medical School and Moderna Therapeutics.

Nearby residents described the town as 'relatively quiet' and expressed shock at the news. 

One told local TV station WWLP:  'It is definitely something scary, being in this town. This stuff doesn’t happen.' 

Another said, 'Just because we do live in Wilbraham doesn't mean that something bad like this can't happen.'

A neighbor close to the murder scene told MassLive, 'It’s a very sad day.'

Police are investigating into when the shootings occurred before the three victims were discovered on Tuesday morning. Pictured: Peter Donaldson