The students are believed to have consumed Dushuqiang, a kind of deadly rat poison, earlier this week in their school breakfasts on Wednesday.

Many students have been allowed to leave the hospital, since they are no longer considered to have been exposed to a lethal dosage, local sources disclosed yesterday.

While some students from Changhu Township Central Primary School are still being kept in hospital wards for further observation, classes have returned to normal in the county, a Yueyang County Party Committee publicity official who refused to give her full name and was only identified as Xiang said by telephone.

"Our public security officials are trying to crack the puzzle as soon as possible," Xiang said, adding that whether the poison was put in the food deliberately was still unclear.

Yang Jian, an official with the county health authority, said that dozens of the students have been dismissed from hospital, but are still being advised to stay under special care at a local teachers' school near the people's hospital of the county.

"The hospital does not have the necessary manpower and facilities for this observation," explained Yang.

"We have placed medical staff at the teachers' school to ensure the students have really recovered for good."

According to Yang, parents of any student can apply to the local education authority to be released from observation, "if they are sure of the healthy condition of their children."

Yang confirmed that over 400 students from the school have visited the county people's hospital since Wednesday, reporting dizzy spells, headaches, nausea or vomiting symptoms. Some were even twitching.

Related medical tests have convinced doctors that the students were poisoned by the notorious rat poison.

(China Daily September 27, 2003)