A Chinese bus driver upset that his home would be demolished had been drinking at the wheel before plunging his vehicle into a reservoir, killing 21 people including students heading to their college entrance exams, police said.

The bus careered into a lake in the city of Anshun in the southwest province of Guizhou last Tuesday. The driver, surnamed Zhang, "was unhappy about his life and about the demolition of the public-owned house he rented," Anshun city police said in a statement on an official social media account Sunday.

Anshun emergency authorities have said five students heading to their university entrance exams were among those killed, while fifteen other people were hurt.

On the morning of the incident, Zhang saw that the house was about to be demolished and rang a government hotline to complain, the police statement said. But police said Zhang had signed an agreement in June authorising the demolition and had been promised more than 72,000 yuan ($10,000) in compensation which he left unclaimed.

Zhang is suspected of deliberately "conducting 'an extreme crime'", the statement said. Police found that Zhang, 52, had been drinking alcohol while passengers got on and off the bus, the statement said.

As he approached the reservoir, Zhang suddenly veered across five lanes of traffic, through a guardrail and into the waters, police said. Zhang drowned and an autopsy found alcohol in his system. A hashtag relating to the Anshun police statement had gained over 1 billion views on Weibo by Monday morning, with many comments condemning the driver. "Why impose your own pain on so many families?" wrote one user.