An advocacy group founded by families of the Sandy Hook massacre victims is celebrating the passing of a new bill that will overhaul the nation’s mental health system.

On Tuesday, President Barack Obama signed into law the 21st Century Cures Act, which includes Connecticut Sen. Chris Murphy’s Mental Health Reform Act of 2016. The bi-partisan legislation aims to help the estimated more than half of all American adults and children with mental health problems who end up going untreated.

Mark Barden, whose son Daniel was one of the 20 first-graders killed in the 2012 shooting at an elementary school in Newtown, Connecticut, is one of the founders of the Sandy Hook Promise group, which worked with Murphy on the Mental Health Reform Act over the past few years.

Barden tells PEOPLE of the bill, “It is going to bring some much-needed, valuable mental health legislation and reauthorization of existing programs that are going to do a lot of good and are going to be a good companion piece to our programs when people are identified as being in need of services.”

He adds, “We have been working closely with Sen. Murphy and other members of Congress — and with a coalition of mental health agencies and with the prevention community — acting as almost a broker between all these organizations to help craft this and to help make sure it gets across the finish line.”

Adam Lanza, the 20-year-old gunman who killed 20 children, six school staffers and his mother before taking his own life in the Sandy Hook massacre, reportedly showed signs of “severe and deteriorating internalized mental health problems” in the years before the shooting, according to a 2014 report based on a comprehensive examination of his medical and school histories. According to The New York Times, the study concluded that Lanza was “completely untreated” for ailments like anxiety and obsessive-compulsive disorder, and was also deprived of recommended services and drugs — despite medical experts at Yale University calling for drastic measures to help Lanza in the years leading up to the shooting.

According to WTNH.com, Sen. Murphy on Tuesday summed up the troubling status of mental heath care in the U.S. by saying, “If your body is broken below the neck you are more likely to get good care than if your body is broken above the neck.”

The Democratic senator, who was in Washington on Tuesday to witness Obama signing his mental health legislation into law, made headlines last June when he launched a nearly 15-hour gun-control filibuster on the Senate floor. His efforts culminated in the majority Republicans’ allowance of votes on two Democratic gun-control measures – one to ban people on the government’s terrorist watch list from obtaining gun licenses, the other to expand background checks to gun shows and Internet sales.

Though the Senate ultimately voted down both measures, Murphy moved many with his emotional Senate-floor speech about the Sandy Hook shooting in his home state of Connecticut.

“For those of us who represent Connecticut, the failure of this body to do anything, anything at all in the face of that continued slaughter isn’t just painful to us, it’s unconscionable,” Murphy said at the time. “I can’t tell you how hard it is to look into the eyes of the families of those little boys and girls who were killed in Sandy Hook and tell them that almost four years later we’ve done nothing, nothing at all to reduce the likelihood that that will happen again to another family.”

Now, the Sandy Hook Promise group is hailing the passing of Murphy’s mental health bill as a major step in the right direction for their cause.

Mark Barden says he’s heard the bill called “the most significant piece of mental health legislation to pass through our federal government in decades — and the most significant legislation to pass through our Congress since the Affordable Care Act.”

It could also be the last piece of legislation President Obama signs into law, he adds. “So we are very excited to be a part of all of this.”