SAR immigration officers are investigating how a mentally handicapped Hong Kong man found by mainland officials in Shenzhen last week managed to cross the border without travel documents. The case comes six months after a similar incident, which involved autistic teenager Yu Man-hon. It has also unleashed further criticism from parents of mentally disabled children who claim the Immigration Department has not learned its lesson. Man-hon, 16, has never been found. The latest incident involves a 22-year-old man who was found at Huanggang control point a day after he was reported missing. The Immigration Department last night said it was still investigating how the man, surnamed Tsoi, 'sneaked into' the mainland on Thursday last week. 'We have no idea how he managed to sneak into the mainland. He was unable to tell us or his family how he ended up in Shenzhen,' an immigration official said. It is understood the department could not find any record of the man leaving Hong Kong. It also remained unclear whether he had crossed over the border at the SAR and mainland passenger checkpoints. The family of Mr Tsoi, whose mental age is unknown, reported him as missing to the police at about 4.45pm on Wednesday last week. He disappeared from his home in Ma On Shan. Shenzhen media reports said Mr Tsoi was discovered in the departure hall of the Huanggang control point at 4.50pm the following day. A mainland border duty officer, Tan Zhengzhuang, found him. Mr Tan thought he might be Man-hon and walked over to the man, who seemed to be looking for a bus. He asked for his identity document but the man said only that he wanted to go to Dongguan. Mr Tan realised Mr Tsoi - who was unable to communicate clearly and logically - was mentally disabled. Mr Tsoi was led into an office where a deputy team leader, Liu Yuqi, tried to question him. When asked where he came from, he pointed to the direction of Hong Kong. When asked where he was going, he said: 'I am going to Dongguan.' Mr Tsoi also scribbled the words 'Dongguan' in complex Chinese characters used in Hong Kong and not the simplified version used on the mainland. Mr Liu believed Mr Tsoi was from Hong Kong, although he carried no identity document. When the mainland border authorities contacted their Hong Kong counterparts to verify the man's identity, Lok Ma Chau immigration control point confirmed that SAR police had received a missing persons report. The description seemed to fit Mr Tsoi. Mr Tsoi was sent back to Hong Kong police and his family was asked to identify him. An Immigration Department spokesman said their close liaison with the Huanggang authorities and the name and telephone number engraved on a pendant worn by Mr Tsoi had enabled his speedy return to his family. The chairwoman of the Parents' Association of Pre-school Handicapped Children, Julie Lee Lau Chu-lai, said: 'The case seems to be similar to that of Man-hon's. How could the Immigration Department allow it to be repeated in just six months?' Man-hon's mother, Yu Lai Wai-ling, said the Government should further tighten border controls to avoid such incidents. Mrs Yu is planning to petition Chief Executive Tung Chee-hwa this morning to ask him to broadcast information about her son on both mainland and local television stations, which can also be watched in Guangdong. Man-hon, who has a mental age of two, ran away from his mother at Yau Ma Tei MTR station on August 24 last year. He was later seen dashing across the Lowu border without a travel document. Shenzhen officers who later found the boy asked the SAR officers to verify his identity. But he was sent back to Shenzhen after the local officers failed to confirm if Man-hon was from Hong Kong. The boy has been missing ever since. Both the immigration authorities and police have improved their handling of missing person cases involving mentally retarded people following Man-hon's case.