In 1955, the number of mentally ill Americans in public psychiatric hospitals peaked at 560,000. Since then, that number has been in sharp decline: as a result of deinstitutionalization and the broad transfer of mental health funding from the federal government to states, people – especially children – living with psychiatric disorders and emotional disturbances have fallen through the cracks.

Liz Garbus’s new HBO documentary, A Dangerous Son, puts this crisis and the country’s inadequate response in sharp focus.

Through the lens of three mothers whose sons suffer from mental illness, the documentary chronicles in intimate, often painful detail the roadblocks families face in securing treatment, as well as the effects of a dismantled and under-resourced apparatus for psychiatric care in the US.

From 2009 to 2012, states slashed funding for mental health services by $5bn while the country got rid of almost 10% of its public psychiatric hospital beds. As the author Andrew Solomon says in the film: “There is the sense that rehabilitation is a luxury.”

Garbus, the prolific documentary film-maker behind Ghosts of Abu Ghraib and What Happened, Miss Simone?, took interest in the subject after the 2012 Newtown shooting, in which 20-year-old Adam Lanza killed 27 people. Though she’d previously examined institutional hurdles and the criminal justice system in documentaries like The Farm and The Execution of Wanda Jean, her hope, this time, was to empathize with the mothers who apply what she calls “herculean efforts” in order to care for their children.

“There was chatter all over the internet and in water coolers: how could a mother not have gotten that son more help? How could she have let him live like that?” Garbus explains. “A woman named Liza Long wrote a blogpost that became very controversial called I Am Adam Lanza’s Mother and she talked about how desperate she had been to get help for her son, how impossible it was to get the care she needed, and how broken the system was.”

In that essay, Long, who appears in the film, wrote: “I live with a son who is mentally ill. I love my son. But he terrifies me.”

The mothers featured in A Dangerous Son, Stacy, Edie and Cora, feel similarly. For the film, they granted Garbus unfettered access to their home lives as they perform the grinding task of caring for their children while jumping through a number of bureaucratic hoops to get them proper treatment. At one point Edie must call the cops on her son, William, 15, after they meet with his social worker; in another scene, Stacy’s 10-year-old son, Ethan, is shown punching his younger sister in the backseat of the car. Later, filming is halted so the camera crew can intervene when Ethan is physically violent toward his mother.

“One wrong word sends me into a psychotic rage,” says Long’s son, Eric. “It’s like watching myself do things I didn’t want to do.”

The specter of mass shootings like the ones in Newtown and Aurora loom large over the film, with one mother expressing fear that she’ll one day see her own son’s face on the news as an example of a child who didn’t get enough help. But Garbus emphasizes the fact that adolescents with mental illness are more likely to be the victims than the perpetrators of violence. “This is not a film about these mass shooters,” she says, underscoring the fact that, while focus on mental health is often used as a crutch in media coverage of gun violence, stigmas about mental illness frequently come at the expense of families like the ones featured in the film.

“People have allowed films to be made about their children suffering from leukemia, and an enormous amount of empathy comes out and donations to childhood leukemia foundations,” Garbus notes. “But when you open your home to somebody whose child is suffering from mental illness, the stigma and the blowback can feel overwhelming.”

As noted in the film, 17 million children in the US have or have had a psychiatric disorder, but there are currently fewer than 60,000 beds to accommodate them. In Virginia and elsewhere, such insufficiencies have had fatal consequences: Austin “Gus” Deeds, the son of the Virginia state senator Creigh Deeds, stabbed his father and killed himself just 13 hours after efforts to secure him a hospital bed fell through. Deeds, who spoke to Garbus for the film, later sued the state of Virginia for medical malpractice and criminal negligence, claiming that his son’s mental health evaluator failed to contact local hospitals that were later revealed to have available beds.

Edie has had similar experiences. “Many times a case worker has no experience with mental illness or autism, and so you have to deal with somebody whose job it is to not give money for the institutionalization of the child,” she tells the Guardian. “Even when the case worker is talking to this child’s therapist who says, ‘Yes, this kid needs intensive, 24/7 care,’ they’re very, very reluctant to do so.”

In remarks made in the aftermath of the school shooting in Parkland, Florida, where 17 people were killed, Donald Trump said the White House was “committed to working with state and local leaders to help secure our schools, and tackle the difficult issue of mental health”. But in his budget proposal for 2019, funding for the National Institute of Mental Health (NIMH) was decreased by 30%, from $1.6bn to $1.1bn.

“There are very limited funds,” Edie says. “To be honest, the battle starts from the moment they start seeing the dollar signs.”

Garbus, whose Trump-themed documentary series The Fourth Estate is being released later this month, says that, despite budget cuts to Medicaid and the NIMH, there are reasons for cautious optimism: last month, a group of three teenagers from Albermarle County, Virginia, successfully lobbied for the state to include $160,000 in its 2019 budget proposal to add a mental health professional to the school system. “These are the kinds of movements that have to happen on a local level for there to be a larger shift in our attention,” she says.

Until that shift takes place, the mothers hope that the documentary will go a long way toward building empathy and understanding for their plight.

“Since he [William] was three years old, I thought I wasn’t parenting the way a good parent would,” says Edie, who co-parents and remains friends with her ex-husband. “That has followed me throughout my journey with him, even though we were a loving family. Even kids from very good homes are susceptible to mental illness and autism and other issues. And that’s a judgment people are quick to go to.”

• A Dangerous Son premieres on HBO on 7 May with a UK date yet to be confirmed