The wreck of a school bus is examined after it collided with a truck on Nov 16,?Yulinzi township, Gansu province. [Xinhua]

ZHENGNING, Gansu - The death toll in a school bus accident in Northwest China's Gansu province rose by one to 21 Thursday night, local authorities said.

A five-year-old boy with severe brain and abdominal organ injuries died Thursday night in a local hospital after treatment failed, according to the county government of Zhengning where the accident happened.

The victims, now including 19 preschoolers and two adults, died when a nine-seat school bus illegally carrying 64 people crashed head-on with a coal truck in Yulinzi township, Zhengning, Wednesday morning. A further 43 were injured.

Hundreds of residents in the province's captial of Lanzhou went to a major city square Thursday night to pay tribute to the victims.

The mourners laid flowers and candles on the steps to the square.

Many were seen standing in silence as passersbys joined.

Li Jungang, chairman of the Little Doctor Kindergarten in Zhengning, was detained on Wednesday for "liability issues," Zuo Jianghua, a spokesman for the Qingyang city government which has jurisdiction over Zhengning, said on Thursday.

Fan Jungang, the driver of the truck that collided with the school bus, was also detained for causing the accident, said Zuo.

Two deputy county chiefs and the heads of the county's education bureau and traffic department have been suspended from duty following the accident.

Parents of students at the kindergarten said school bus overloading has been a problem for years, despite repeated complaints.

An initial investigation showed that the school bus had its seats removed to make room for more passengers and was speeding in adverse weather conditions, Zuo said.

The accident has shocked the nation. Following it governments in several provinces and cities have already demanded comprehensive safety inspections of school buses.

Lu Huadong, chief of the Education Bureau of Qingyang, admitted that the city, a poverty-stricken area, had a limited education budget for preschool education.

According to Lu, the city has only some 40 public kindergartens and some privately-run kindergartens often ignored safety rules to overload school buses in order to reduce costs.

The Little Doctor is the only kindergarten in Yulinzi township. With over 700 children, the privately-run kindergarten has only four vans. All seats have been removed so more passengers can be crammed in.

"Merely three days before the accident, we discovered overloading of kindergarten school buses and ordered rectification, but the operators just ignored our request for the sake of profit, " Lu said.

In Zhengning, many privately-run kindergartens have emerged to look after preschoolers as many of their parents work away from home in cities.

"In the beginning we sent and picked up our kids ourselves by bicycle. Later private kindergartens vying for business started school bus services to enroll more children," said a local resident who did not reveal his name.

"Most of the 'buses' are actually vans, which are frequently overloaded," said the villager.