Police in China have arrested nine people in relation to explosions that killed 55 people, state media report.
The blasts in the eastern city of Qingdao took place on Friday, after a leaking oil pipeline owned by oil refiner Sinopec caught fire.
Seven of those arrested were Sinopec employees, while two came from the city's economic and technological development zone, Xinhua reported.
The explosions injured at least 160 people, with nine people still missing.
The blasts took place in two locations after oil leaked from the pipeline caught fire, media reports said.
Yang Dongliang, director of the State Administration of Work Safety, led a preliminary investigation into the incident and said that there had been "a serious lapse of responsibility", state media said.
The incident "revealed problems, including a poor layout of the oil pipelines and the city's drainage pipes", Xinhua quoted Mr Yang as saying.
"The pipe was not properly maintained, leading to the original oil leak, and following the leak the emergency procedures were insufficient. They did not take preventative measures such as sealing off the area and evacuating people," Xinhua added, quoting Mr Yang.
Pictures of the blast showed concrete slabs and roads shattered, and overturned cars.
More than 100 firefighters were deployed to put out the fire.
On Saturday, Sinopec apologised for the explosions and said it would "investigate the incident with responsibility".
Chinese President Xi Jinping visited survivors of the blast at a Qingdao hospital on Sunday.