At least two people were killed after an explosion rocked the port city of Ningbo in eastern China’s Zhejiang province on Sunday, causing buildings to collapse. The explosion, which occurred at about 8.50am, shattered windows up to a kilometer from the blast site. The cause of the explosion was still unknown around 10 hours after it took place, according to the South China Morning Post. In addition to the two deaths, another two people were “seriously injured” and 16 suffered minor injuries, China’s state-run CCTV reported. While the cause of the blast is still under investigation, it is the latest in a series of incidents that have raised concerns about safety in China’s manufacturing sector. In August 2015, two blasts ripped through the industrial Binhai New Area in the port city of Tianjin, killing 173 people – including firefighters – and injuring hundreds. Another explosion in a residential complex in Fugu county, Shaanxi province, in October last year killed at least 14 people.