NINGBO, Zhejiang -- Police have ruled out human factors in a deadly explosion in East China's Zhejiang province on Sunday, which killed two and injured 19.

Technicians from the local public security department said Monday that human factors had been ruled out as a cause, but further investigation is still under way to find out what caused the explosion.

The explosion occurred at the site of a demolished factory around 8:50 am on Sunday in Jiangbei district of Ningbo city. Two people were confirmed dead and 19 were injured, including four with serious injuries, while two others went missing.

After four rounds of search and rescue, firefighters have yet to find any people trapped at the disaster site.

Affected residents have been relocated to two sites. Maintenance work has started in nearby residential communities. Affected residents are expected to receive subsidies after evaluation.

Previous investigation confirmed that the center of the explosion was a septic tank, and excluded the possibility of a piped gas explosion.