The man accused of fatally stabbing a young Ukrainian refugee aboard a North Carolina train erupted in court on Tuesday, screaming at a federal judge moments before he was formally declared unfit to stand trial.

Decarlos Brown Jr, 35, repeatedly interrupted proceedings during a hearing in Charlotte, demanding to know whether the judge had reviewed his mental health evaluation and insisting he wanted to press charges against the FBI.

Brown continued shouting as court officers moved to escort him from the courtroom, making remarks about having 'material in his body' and asking whether the judge had received a letter from his mother.

The outburst came as a federal judge ruled that Brown is mentally incompetent to stand trial in the killing of 23-year-old Iryna Zarutska, a Ukrainian refugee, last year.

Brown is charged federally with causing death on a mass transportation system, an offense punishable by death. 

He also faces a separate state charge of first-degree murder, although that case has been paused while federal proceedings move forward.

US District Judge Kenneth D Bell ordered Brown committed to a federal medical facility for treatment after mental health experts concluded he is presently unable to understand the legal proceedings against him or properly assist in his own defense.

According to court filings, Brown has repeatedly claimed he is suffering from what he calls a 'body emergency' and believes someone has gained control of his body through technology that authorities refuse to investigate.

Defense attorneys told the court that Brown wanted the judge to hear a statement alleging that 'someone has full access to my body and they are controlling me wrongfully' and that he had been wrongly diagnosed with schizophrenia.

A forensic mental health evaluation conducted by federal examiners concluded that Brown 'is presently not competent to stand trial,' according to the judge's order.

However, experts also determined that his condition could potentially improve with treatment.

The court found Brown's prognosis for restoration of competency was favorable if he received appropriate medication and medical care.

As a result, Bell ordered Brown transferred into the custody of the US Attorney General for hospitalization and treatment for up to four months.

After that period, the court will reassess whether Brown is capable of standing trial, whether additional treatment is necessary, or whether competency cannot be restored.

Outside the courthouse, US Attorney Russ Ferguson said the ruling was an important step in the legal process.

'The court found that his prognosis is "good" to be made competent and set a four-month deadline to have that process go forward,' Ferguson told reporters.

'The court ordered that he seek treatment and medication,' Ferguson added, noting that Brown will remain in federal custody while undergoing treatment.

Ferguson said prosecutors remain focused on obtaining justice for Zarutska and her family.

'Our number one goal here is justice for Iryna Zarutska and Iryna Zarutska's family,' he said. 'That's what's on the top of our minds and our hearts every day, and this is a step in that process.'

Asked about Brown's courtroom behavior, Ferguson said it was difficult to determine exactly what the defendant was attempting to communicate.

'It wasn't entirely clear what he was yelling out,' Ferguson said. 'We've seen since the beginning of this case, he says there's some sort of material that controls him and I think it was related to that. And that plays a little bit of a role in why he was found not competent to stand trial.'

Authorities say Zarutska was riding the train, returning home from her job at a pizza restaurant, when a crazed attacker snuck up on her from behind and stabbed her to death on August 22 last year.

Footage of the attack on Zarutska showed the refugee sitting by herself, looking down at her phone as Brown boarded the train and sat behind her. 

Moments later, Brown appeared to unfurl a switchblade before standing up and looming over the refugee. 

The refugee who fled her war-torn home in 2022 bled to death with fear in her eyes as onlookers did nothing to intervene. 

Prosecutors identified the cold-blooded culprit as Brown, who blamed his violent actions on a schizophrenic episode. 

Zarutska, who had escaped war-torn Ukraine after Russia's invasion and resettled in the United States, died from her injuries shortly afterward.

Brown was arrested soon after the attack and has remained in custody ever since.

He was previously found incompetent to stand trial in the parallel state murder case, while a separate evaluation conducted by the Department of Justice's Bureau of Prisons also concluded that he was not competent to face the federal charges.

Questions over Brown's mental state come after the Daily Mail exclusively revealed Brown's first-ever jailhouse calls after allegedly murdering Zarutska, in which he spoke of 'materials in my brain' which he claimed forced him to stab her. 

Brown had dozens of prior arrests before Zarutska's murder and had been freed by North Carolina Magistrate Judge Teresa Stokes just seven months prior.

The killing sparked calls for tougher sentencing for repeat offenders after it emerged that Brown, whose previous arrests included armed robbery and assault, was freed on a 'written promise' that he would return for his next court appearance.

In her obituary, Zarutska's family said that before starting her new life in the US, Iryna graduated from Synergy College in Kyiv with a degree in Art and Restoration. 

She was described as an adventurous young woman who adored animals and traveling. 

'She shared her creativity generously, gifting family and friends with her artwork,' they said.