The father of a Sandy Hook Elementary School shooting victim has been found dead, having apparently taken his own life, police say.
Newtown, Connecticut, police said Jeremy Richman was found in his office building on Monday.
The 49-year-old was the father of six-year-old Avielle, one of 20 children killed in the December 2012 shooting.
Police said they expect a post-mortem examination from the medical examiner later on Monday.
"This is a heartbreaking event for the Richman family and the Newtown Community as a whole," Newtown Police spokesman Lt Aaron Bahamonde said in a statement.
"The police department's prayers are with the Richman family right now, and we ask that the family be given privacy in this most difficult time."
The department will not reveal "the method or any other details of the death" except that it "does not appear to be suspicious".
In the wake of the Sandy Hook shooting, one of the deadliest in US history, Mr Richman and his wife, Jennifer Hensel, started the Avielle Foundation, which seeks to prevent violence through brain science research, community engagement and education.
In a statement, the foundation said Mr Richman was "a champion father, husband, neuroscientist and, for the past seven years, a crusader on a mission to help uncover the neurological underpinnings of violence".
"Tragically, his death speaks to how insidious and formidable a challenge brain health can be and how critical it is for all of us to seek help for ourselves, our loved ones and anyone who we suspect may be in need."
Mr Richman had been advocating for fixes to the mental health care system, and left his job as a pharmaceutical researcher to pursue that mission, the Hartford Courant reported.
Last week, he had spoken at Florida Atlantic University about how educators could identify mental illness and prevent school shootings.
Ron Conway, an investor involved with Sandy Hook Promise, another non-profit gun violence prevention group that formed after the attack, launched a GoFundMe to raise money for the Avielle Foundation.
Starting with a $100,000 (£75,000) donation from Mr Conway on Monday, the fundraiser has received over $118,000 as of Tuesday morning.
The news of Mr Richman's death follows the apparent suicides of two teenage survivors of the February 2018 Parkland, Florida, school shooting.
A total of 17 students and teachers died in the attack at Marjory Stoneman Douglas High School.
Nineteen-year-old Sydney Aiello took her own life last week after suffering from survivor's guilt and post-traumatic stress disorder, according to CBS Miami.
She had been a high school student at the time of the shooting and lost her friend, Meadow Pollack, in the attack.
A second Stoneman Douglas pupil apparently took his own life on Saturday, police say. They have not named the student.
Broward County Schools Superintendent Robert Runcie said on Sunday that parents and organisations across the county were discussing what could be done to help children "throughout the county cope with trauma and depression".
Where to get help
From Canada or US: If you're in an emergency, please call 911
You can contact the US National Suicide Prevention Lifeline on 1-800-273-8255 or the Crisis Test Line by texting HOME to 741741
Young people in need of help can call Kids Help Phone on 1-800-668-6868
If you are in the UK, you can call the Samaritans on 116123