Three police officers and a town official are being held in custody after allegations they abandoned a badly injured teenager in a ditch because they thought she was a mentally ill vagrant.

The girl surnamed Wu, 18, a senior at No 4 Guoyang County Middle School in Anhui province, was spotted lying half-naked and semi-conscious in an alleyway by a passer-by.

The police officers in Gaogong town, Guoyang, who responded to the emergency 110 call, believed Wu was "homeless and in a confused state of mind", and decided to refer the matter to Zhou Yuzhu at the Gaogong civil affairs bureau, authorities said.

Instead of transferring Wu to hospital, however, Zhou put her in a car of the civil affairs bureau and told the driver to dump her in the neighboring county, Taihe. Local media reported that she was later discovered in a ditch by passers-by and rushed to Taihe People's Hospital, where doctors discovered she had a fractured skull.

"The officers from Gaogong town police station didn't carry out further investigations on the spot. Instead, they assumed the teenager was a homeless person with a clouded mind and called the civil affairs department to pick her up," said Deng Wei, a spokesman for the Guoyang public security bureau.

"From what we can gather, she was actually attacked on her way home from school," he said.

An official in the Gaogong civil affairs bureau who gave his surname as Li, said: "Zhou should have confirmed the woman's identity and driven her to a hospital, but he didn't."

He said the police did not mention any suspicious circumstances when they called his colleague about the girl, and added that Zhou had only started in the job in February after Spring Festival.

Wu remains in the intensive care unit at Taihe People's Hospital.

"I've been waiting outside ICU since the night of March 13," said Wu Jian, the teenager's father, who had returned from Shanghai, where he works.

Zhou and the police officers involved have been detained on suspicion of dereliction of duty and the incident is being investigated.

However, a police officer in Shanghai, who did not want to be identified, said the men involved faced a more serious charge than dereliction of duty.

"The policemen should take care of anything happening in their area, but they just transferred the case without proper scrutiny," he said.

It was worse for the man in the civil affairs department, he added. "It could be a charge of manslaughter if the man abandoned a person in a coma."

Yang Chao, a Beijing-based lawyer with Huawei Law Firm, agreed and said the four men could be charged with willful injury or even homicide, if the girl died, because they "transferred the girl from their jurisdiction to another's and contributed to possible death".

Zhou Wenting in Shanghai and Liu Nanxue in Hefei contributed to this story.