The Anonymous hacking collective routinely targets corporations, famous figures or financial institutions in the name of social justice. But what would lead this notorious group of politically-charged digital activists to target a children's hospital?

Now, for the first time, a hacker allegedly closely involved in a 2014 campaign called OpJustina has come forward to elaborate on the reasons for a controversial series of distributed denial of service (DDoS) cyberattacks against a number of child treatment centres in the US.

"The answer is simpler than you might think: The defence of an innocent, learning disabled, 15-year-old girl," Martin Gottesfeld, 31, told the Huffington Post in a lengthy statement from federal custody, where he is facing computer misuse charges relating to the attacks. "In the criminal complaint she's called 'Patient A' but to me, she has a name, Justina Pelletier."

Justina was 15 years old when first admitted to the Boston Children's Hospital. At the time, in 2013, doctors ruled that a mitochondrial disease she was being treated for was largely psychiatric in nature and as a result she spent 16 months in a mental health ward as her parents fought to regain custody.

Her father, Lou Pelletier, who is now taking the Boston hospital to court, has previously claimed she suffered torture and abuse while under state care.

"They stopped her painkillers, leaving her in agony. They stopped her heart medication, leaving her tachycardic," claimed Gottesfeld. "They said she was a danger to herself, and locked her in a psych ward. They said her family was part of the problem, so they limited, monitored and censored her contact with them."

The Anonymous member said the hacktivist campaign was swiftly launched to help bring coverage to the case. The group released a video announcing the launch of the operation and publicly called on Boston Children's Hospital to fire the physician who diagnosed Justina.

"All other efforts to protect Justina weren't succeeding and time was of the essence," Gottesfeld said. "I'd have to hit BCH where they appear to care the most, the pocket book and reputation. Almost unbelievably, they kept their donation page on the same public network as the rest of their stuff. Rookie mistake. To take it down, I'd have to knock the whole hospital off the internet."

The attacks on the website, which sent waves of traffic towards its servers in order to force it offline, were effective. According to Gottesfeld's arrest affidavit, the cyberattack took down the hospital's computer network for at least a week and caused $300,000 (£230,000) worth of damage.

Yet Gottesfeld claimed the attacks resulted in minimal harm – especially on other patients, a concern that caused vocal backlash even within the ranks of Anonymous at the time.

"Almost unbelievably, they kept their donation page on the same public network as the rest of their stuff. Rookie mistake. To take it down, I'd have to knock the whole hospital off the internet."

- Martin Gottesfeld

"No patients should be harmed if Boston Children's was knocked offline," he said. "There's no such thing as an outage-proof network, so hospitals have to be able to function without the internet. It's required by federal law, and for accreditation. The only effects would be financial and on BCH's reputation.

"I had spent my career building cyber defences. For the first time, I was on the offensive," he continued. "I coded around the clock for two weeks to perfect the attack. Small test runs were made. BCH bragged to the media that they were withstanding the onslaught and hadn't been taken down. They had no idea what was to come.

"I finished the code just in time. It ran. BCH's donation page went down. Then, with some donation time still left, I issued the command to stop the attacks - the point had been made. Justina wasn't defenceless. Under the banner of Anonymous, she and other institutionalised children could and would be protected."

Gottesfeld's intentions may have been pure, but from the perspective of the FBI they were a federal crime. In February this year, the hacker was arrested by US authorities in Miami after going on the run with his girlfriend. He stands accused of playing a heavy role in OpJustina, including being the one who posted the Anonymous video to the internet calling for mass protest. He faces five years in prison.