A non-verbal black teenager was arrested by immigration officers because he gave police the wrong name and address.
The 17-year-old, from Cranbrook, disappeared from a hospital after being treated for mental health issues and was found at an immigration detention centre days later.
His family has accused British Transport Police (BTP) of racial profiling and said officers "saw his race" before seeing a vulnerable teenager.
The boy, who KentOnline is choosing not to name, went missing on Thursday, April 7, after being sectioned under the Mental Health Act, and was arrested by BTP officers two days later at Euston on suspicion of fare evasion.
He was then detained by Immigration Enforcement near Gatwick, despite being British.
It is uncertain where the teen was in the days after he left the hospital but it's thought he tried returning to the family's old home in Manchester before travelling back to London.
His sister claimed online her brother, who struggles to communicate and had no ID or money, was found by officers walking bare foot in the rain.
"He had lost his sliders and tried to get back to London in the cold, wet rain on the third day," she said.
'If they had met a white teenage minor who was barefoot and distressed, would their first instinct be to deport him? Of course not...'
"On the way back home he was met by British Transport Police who saw that he was in visible distress.
"Instead of assuming that this is a person who required help and was in pain, they arrested him, bundled him into a van and took him to a detention centre.
"Because he had no form of identification and couldn't speak they transferred him to Gatwick airport to be deported. They thought he was an illegal immigrant.
"They locked him up like a prisoner and mishandled him. Where were they going to deport him to I ask?
"A boy born in the UK who has never set foot in another country was going to be illegally deported all because he couldn't talk."
She went on to claim if he had been a white teenager, the incident would've been dealt with differently.
She added: "If they had met a white teenage minor who was barefoot and distressed, would their first instinct be to deport him? Of course not.
"But when they saw my brother they didn't see a boy in pain, they saw his race.
"He would have been lost and we probably would have never found him.
"I am angry and disturbed that there are families who have lost loved ones, or that are still missing and will never know."
British Transport Police have said Immigration Enforcement were called after the boy allegedly gave a false name and age.
A spokesman said: “British Transport Police received a report of a fare evader on-board a train from Manchester to London at 8.54am on April 9.
"Officers attended Euston station and met the male who provided a false name and date of birth, stating he was 20-years-old. He was subsequently arrested on suspicion of fare evasion and taken to police custody.
"In custody, officers were unable to engage with the male or verify the details provided despite their efforts, which included attending the address provided.
"As is procedure, fingerprints were taken which include checks against Immigration Enforcement. Given the information provided, they attended custody and detained the male.
"A senior officer has reached out to the male’s family to explore any concerns around the incident.
'Our thoughts are with him and his family during what has been a difficult time...'
"No further action was taken against the male with regards to the fare evasion, and he was returned home safely."
The teenager was receiving care from the NELFT NHS Foundation Trust when he went missing.
A spokesman said: “Our thoughts are with the patient and his family during what has been a difficult time.
“We are unable to provide specific comment on individual cases in the interest of patient confidentiality.
“The Trust works proactively with a multitude of services to ensure the safety and security of patients.
"As an organisation we are reflecting on his experience as we continue to provide patient support and care.”
The Home Office has been approached for more information.