GUANGZHOU - Law enforcement authorities in Maoming, a coastal city in South China's Guangdong province, are investigating a case where a young detainee died mysteriously in a detention facility late last month.

Qi Yeqiang, a 22-year-old inmate in the Maoming No 1 Detention House, was found unconscious in the early hours of Nov 25. He was then sent to the Maoming People's Hospital, where he was pronounced dead.

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Doctors said Qi died before he was rushed to the hospital.

According to Qi Yi, uncle of the deceased, a police officer explained that the young man suffocated after his head was covered by a quilt while he was sleeping.

Qi Yeqiang, a resident in the city's Maonan district, was detained on April 28 for alleged involvement in theft. He was officially arrested on June 4.

Qi's family said he had been in good health and suspected that prison authorities were involved in the death, information portal dayoo.com reported.

After the family members filed their suspicions on Qi's death, the city's law enforcement authorities established a team to investigate the case and invited experts from the medical college of Guangzhou-based Sun Yat-sen University to conduct an autopsy.

Qi was found to have no wounds, ruling out the possibility of Qi's being tortured in the detention house, said a local police officer who refused to be named.

"The doctors found plenty of blood in Qi's lungs, indicating he was suffocated," he added.

But the police's explanation was quickly questioned by Qi's family members, local residents and many netizens.

Qi Shuijin, older brother of the deceased, said he did not believe the police's explanation.

"My brother was very healthy and he wouldn't get sick, unless something happened," he said.

"We can't accept the police's explanation that Qi had been suffocated while sleeping," he added.

Many netizens also refused to believe Qi died of suffocation.

"It is impossible that a young man died just because he was covered by a quilt while sleeping," said a netizen surnamed Cheng.

Police authorities introduced stricter monitoring measures in the country's jails last year after at least 15 people died in unusual circumstances in the first few months of 2009 alone.

Police have cited a litany of suspected causes of death including "falling out of bed", "suddenly drinking cold water" and "playing hide-and-seek with other inmates", earlier reports said.