Four children in Southwest China's Guizhou Province died after drinking pesticide in a tragedy that once again sparked concerns over left-behind children in rural areas in China.

The siblings, a boy and three girls aged 5 to 13, died after swallowing pesticide on Tuesday at their home in Bijie, Guizhou, one of the poorest areas in the mountainous province.

News website caixin.com reported that an hour or so before their death, the four were visited by local school teachers and education officials, who persuaded the 13-year-old boy who has quit the school for weeks to return to class.

Several visitors reached by the website claimed that they did not see any pesticide during the visit.

Village official Hu Haifeng, one of the visitors, said that he and several school teachers attempted to visit the children several times since three of the four quit school in early May. The boy promised to return to school, Hu said.

On Friday, police said the boy had left a letter behind. "Thank you for your kindness … I had sworn not to live after 15 and death has been my dream all these years. I've been planning it for a long time and today is the time," the letter read.

Five local officials have been suspended or removed from their posts. Bijie has formed an investigation team to further look into the case.

According to the Xinhua News Agency, the children had not been contacted by their mother for over a year. She has been working in Guangdong Province and only returned to Bijie on Friday. Their father also left them in March.

Villagers said that the four children had been diffident and isolated, The Mirror reported.

This is an extreme case that reflects the problems confronted by the 60 million-plus children in rural areas that are left behind, as the country's urbanization drive draws millions of parents away from their rural homes to cities for work, observers said.

According to a 2013 report released by the All-China Women's Federation, nearly 3.4 percent of all left-behind children live alone, often falling victims to tragedies such as suicide and human traffickers.

In 2012, five street children - also from Bijie - died from carbon monoxide poisoning after burning charcoal to get warm in a roadside dumpster. 

Wang Xingjuan, a Beijing-based psychologist, said that lacking of parental care would cause many mentality issues and lead children to extreme acts.

Many migrant workers don't take their children with them to cities because without a city hukou, or house registration, their children cannot enroll in local schools.