Douglas Butler was confused. He thought his son, Andrew Butler, had been transferred to another, more secure psychiatric hospital. But when he drove to the address of the secure psychiatric unit on the outskirts of New Hampshire’s capital of Concord, he couldn’t see anything that resembled a hospital.

Instead, he saw the coils of razor wire and imposing walls of the New Hampshire state prison for men.

“They claim it’s a psychiatric hospital. But it’s nothing of the kind,” said the older Butler. “It’s a dang prison. That’s all it really is.”

Butler, then 21, had been charged with no crime. But for the mentally ill deemed too much of a danger to themselves or others to remain at New Hampshire hospital – the state’s lone psychiatric hospital – that can mean a transfer to the secure psychiatric unit (SPU) at the prison, where Butler was dressed in a prison jumpsuit and kept in solitary confinement.

According to mental health advocates, New Hampshire stands alone in the US in warehousing involuntarily committed mental health patients in prison. There have been efforts to reform the system for years now, but the state’s legislature never agreed on the millions of dollars needed to build a secure psychiatric facility, leaving the prison SPU as the only option for some patients.

The New Hampshire department of corrections has defended the unit’s use, saying patients receive the same level of care as they would at a psychiatric hospital and that patients get “exceptional treatment”.

But inside the SPU, patients are dressed like prisoners. They are kept locked in cells equipped with steel toilets and slabs serving as beds. Cell doors have slots to receive food. Their neighbours can be convicted felons. Guards patrol halls. Visitors are restricted. Phone calls are recorded. Mental health advocates pushing for reform say medications deemed valuable in the illicit prison economy are forbidden. Group therapy sessions are conducted in cages. Like inmates, patients can be subjected to use of force. There is no illusion of freedom.

It is a stark contrast to a traditional psychiatric hospital setting.

“They are in probably the worst place you can imagine to be with a psychiatric crisis,” said Frankie Berger, director of advocacy at the Arlington, Virginia-based Treatment Advocacy Center.

Butler’s journey to the New Hampshire state prison for men began on a trip to Vermont with friends in the summer of 2017. As college kids sometimes do, they took hallucinogenic drugs. But the effects seemed to set something off in Butler. He was hospitalised in Vermont and when he returned home to New Hampshire, he wasn’t the same. He told his father it felt like he was not in control of his actions. Eventually, law enforcement officers found him running through the forest punching trees.

The treatment he has received is cruel and unusual punishment without having been convicted of a crime. Sandra Bloomenthal

In a move his father supported, Butler was civilly committed to New Hampshire hospital to receive psychiatric treatment.

After a few months, Douglas said Butler started becoming increasingly aggressive with the doctors. And at one point while he was visiting, Butler took a swing at him. Douglas said it wasn’t serious, just “a tap on the nose”.

Hospital staff did not view the incident so lightly. They deemed Butler a significant threat to others and he was transferred out of New Hampshire hospital and into the SPU.

In a habeas corpus petition filed last year, Butler’s lawyer Sandra Bloomenthal wrote: “He is held in a maximum security prison. He is held as a mental health patient without being in an accredited hospital, denied contact visits with his father, denied contact visits with his attorney, forced to wear prison clothing. He is locked down 23 hours a day. He has been tasered.”

She added: “The treatment he has received is cruel and unusual punishment without having been convicted of a crime and with no pending criminal process.”

It remains unclear how many civilly committed patients have ended up in the SPU over its more than three decades of existence. The New Hampshire department of corrections did not grant the Guardian’s requests for interviews and a tour of the facility, nor did they respond to a list of questions.

In testimony from 12 February, the SPU director, Paula Mattis, said of 41 beds occupied, 32 “are individuals whose transfer to the SPU emanated from the justice system”.

In an interview with the Guardian, the New Hampshire hospital CEO, Lori Shibinette, said there had been seven transfers of patients from her facility to the SPU last year. She said all of those transferred last year had been returned to New Hampshire hospital and that there had been no transfers so far this year.

Those transferred, Shibinette said, were typically patients who displayed repeated, unpredictable or premeditated violence along with those whose self-harm attempts could not be controlled. One patient she described would attempt to ingest “anything she could get her hands on” from ceiling tiles to pens and screws.

Shibinette said the SPU offered the same level of care as New Hampshire hospital, just in a more secure setting.

“SPU is a much more controlled environment,” she said.

While Shibinette stressed the SPU is a temporary measure, some patients have been there for years. One patient, Anthony Heath, was civilly committed in 2016. His family says that after he made threats towards New Hampshire hospital staff, he was sent to the SPU, where he has remained for nearly three years now.

The overall number of transfers appears small enough that the SPU program largely avoided attention within the state and across the nation.

Berger first heard of the SPU a few years ago, when a New Hampshire bill aiming to end the practice of sending civilly committed patients there was passed along to her. As she read through the document, she thought there was a misunderstanding.

“I thought there’s no possible scenario – because it’s so unconstitutional – that what this bill is being used to address is a reality,” she said.

You could find yourself stuck there for months or years longer than anybody thinks you need to be there. Andrew Milne

The nature of the SPU was also a surprise to nurse Beatrice Coulter, who took a job there in 2015 with the understanding that she would be treating prisoners. Her employment in the SPU lasted only days, as she decided to quit once she realised that there were civilly committed patients incarcerated there.

“I was absolutely stunned when I realised that there was a whole population of patients there who were not inmates,” she said. “I did not want to be a part of this.”

After her stint at the SPU, Coulter teamed up with the criminal justice reform activist Wanda Duryea to found Advocates for Ethical Mental Health Treatment, a group that brought together families of civilly committed patients in the SPU and pushed for reform.

New Hampshire Disability Rights Center staff attorney Andrew Milne said that few seeking psychiatric help in the state probably realise that they could potentially end up in prison.

“I think it would be a big surprise to most people that you could be admitted to the state hospital because of symptoms of mental illness and find yourself transferred to the prison and treated as an inmate – and then find yourself stuck there for months or years longer than anybody thinks you need to be there,” he said.

Now, more than 30 years after the SPU was established and a law was passed allowing civilly committed patients to be sent there, the days of the practice may soon be numbered.

Butler’s case struck a chord in the state, prompting protests and previously unseen press coverage as a lawyer sued for his release.

In June, Butler was transferred back to the New Hampshire hospital after nearly half a year in the SPU. Shortly after, he was allowed to return home.

The New Hampshire hospital in Concord. Photograph: Josh Wood/The Guardian

The renewed spotlight on the SPU saw calls to take action to see other civilly committed patients released. Soon, the Republican governor, Chris Sununu, signalled his support for ending the transfer of civilly committed patients to the SPU.

In presenting his budget last month, Sununu proposed a 60-bed secure psychiatric facility to be built at New Hampshire hospital, a move that would remove civilly committed patients from the prison.

“Moving the civilly committed population in the SPU out of the state prison has been an issue-in-waiting for several decades. It is time to get it done – and that is what I am going to do so we can treat our patients with the dignity they deserve,” the governor told the Guardian.

But after Butler’s ordeal, his father remains untrusting of the state. He lost guardianship of his son during the ordeal and now questions the healthcare he receives at the orders of a state that incarcerated his son.

“In a way now we’re still kind of in prison as we don’t have control over his mental health treatment,” he said.