Police Monday detained a jobless resident suspected of setting up a bomb at a chain restaurant Sunday in Northeast China's Heilongjiang Province.
The motive of the crime is not yet known, the police said.
The 34-year-old used black powder disassembled from firecrackers to make the bomb, and set it up Sunday morning at a Dicos restaurant in Anda of Suihua City, Heilongjiang, the police said.
Zhu made an anonymous phone call to the restaurant's delivery hotline, claiming that there would be an explosion at the restaurant within five minutes, the police said, adding that he blackmailed the restaurant for 100,000 yuan ($16,000).
Sheng Wei, head of the public security bureau of Suihua, said police received an anonymous phone call claiming that there would be an explosion in the restaurant within minutes.
Police rushed to the scene to evacuate people. The explosion occurred during the evacuation and three policemen were injured, according to local authorities.
Police found an explosive device was placed at the corner of the staircase to the second floor.
Heilongjiang Morning News cited witnesses as saying that before the blast a man walked into the restaurant and left a bag without buying any food. A restaurant employee then received a call claiming there was an explosive device.
Two of the injured officers, Wang Jiliang, 25, and Zou Defeng, 37, have been transferred from a local hospital to the Harbin No 5 Hospital in the provincial capital to be treated for serious burns.
This is the fourth major criminal act involving explosive devices in the past month.
Two people were injured by a blast in Chongqing's Tongnan county on Wednesday morning. Local police said a man with marital problems who had access to explosives at work was responsible for the blast.
On May 22, terrorists drove two vehicles into people at a market and threw explosive devices into the crowd in Urumqi, the Xinjiang Uygur autonomous region. Thirty-nine people were killed at the scene or later died, and 94 were injured.
Dai Peng, director of the criminal investigation department under People's Public Security University of China, said: "Compared with other criminal acts, an explosion can cause higher casualties and inflict a stronger impact on the public's mentality, so some people wanting to take revenge on society are more likely to resort to it."
Zhou Huiying in Harbin contributed to this story.