Chinese authorities said Friday an explosive device made by a 22-year-old local man was behind a deadly incident outside a kindergarten in the eastern part of the country.

The man, surnamed Xu, was killed at the scene of Thursday's explosion, they said, adding that investigators found materials for making an explosive device in his rented room.

The blast at the front gate of the kindergarten on the outskirts of Xuzhou, a major city in the province of Jiangsu, killed at least eight people including the suspect and wounded 65 others.

It occurred just as parents and others were waiting to pick up children. The authorities said eight remained in critical condition.

The dead man from the Chinese city was living near the kindergarten and had been working part-time after his schooling was cut short due to autonomic imbalance, according to the authorities, who offered no possible motive for the crime.

The walls of Xu's room were covered with such words as "death" and "extinction," they said.

The authorities are looking into the deadly explosion as a criminal case, not an accident.

Videos purportedly from the scene posted Thursday night on social media and online news sites showed a number of small children and adults lying on the ground, with some bleeding and their clothes torn apart.

But the authorities said no children or teachers from the kindergarten were among the casualties as they were still inside the nursery school at the time of the explosion.

More than 100 police officers were seen deployed outside the kindergarten early Friday, continuing with on-site investigations.