A family has lodged a formal complaint against the Met Police after their teenage son, who has Down’s syndrome, was arrested and detained for nine hours after trying to collect his favourite baseball cap from school on bank holiday Monday.

Officers issued 19-year-old Abdul Al-Faisal with a caution for burglary over the incident in which he set off an alarm and was caught, with the cap in his hands, at 10am yesterday.

Abdul’s mother Roshina Al-Faisal told the Evening Standard how her son has a mental age between 10 and 12 years old and did not understand what had happened. She said she arrived at the police station to find her son “terrified” and in tears.

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The family say Abdul was handcuffed, had his fingerprints taken, swabbed for DNA and had his details put on record – all while his parents did not know where he was.

He was only discovered after they gave up searching the streets around their home in Hornsey, North London and called the police to report him missing.

Ms Al-Faisal said: “He doesn’t realise the situation. He is shaken up and so distressed. He has never been in trouble before.”

Abdul is believed to have left his home on his own and walked three miles to Haringey Sixth Form Centre in Tottenham where he studies creative arts, climbed in through an open window and triggered an alarm.

When police arrived they checked CCTV and scoured the premises to find Abdul inside, having located his cap.

After being arrested at 10am, Abdul remained in a cell until 7.30pm when he was released. His family say he shouldn’t have been questioned without them there and that he was “treated as a criminal”.

His father, Muhammad Al-Faisal, said: “When they found out my son had Down’s Syndrome it should have been dismissed immediately. We said they were making a terrible mistake.”

A police spokesman told the Standard Abdul had been treated as a vulnerable person and correct procedures were followed.

He said: “Police were called at shortly before 10am yesterday following an alarm activation. Officers attended and found evidence suggesting a break-in.

“A check of CCTV identified a suspect and a search of the premises found a male on the premises. The male was arrested on suspicion of burglary and taken to a north London police station.

“The male arrested, aged 19, was noted as being a vulnerable adult and safeguards provided for vulnerable detainees by the PACE Code of Practice were followed. He was later given a caution for burglary and released.”