It has been learned that a man who was detained in southern China for allegedly stabbing a Japanese schoolboy to death has been formally arrested.

Sources familiar with Japan-China relations revealed the news.

A 10-year-old boy was assaulted on his way to a Japanese school in Shenzhen, Guangdong Province, by the knife-wielding man in September. The student died of his injuries. The suspect, who is in his 40s, was taken into custody at the scene.

Japanese officials are asking Chinese authorities to explain the assailant's motive, but no information has been provided so far.

The incident has forced Japanese schools in China to take safety measures. They plan to close or hold online classes on December 13, which China designates as the day to mourn the victims of the Nanjing Incident.

The Chinese government says atrocious acts were committed after the former Japanese Army entered Nanjing in 1937. The Japanese government says it is undeniable that the killing of noncombatants occurred in the Nanjing Incident. But it maintains it is difficult to determine the correct number of victims.