The death toll from the Friday blast, meanwhile, rose to 55. Another nine remained missing as of Tuesday, China Daily reported. The blast at the pipeline, owned by Sinopec, China's largest refinery, also injured more than 160 people, who were taken to hospitals. Qingdao is in eastern China's Shandong province.
It was not clear if any charges had been filed against the nine in police custody. The other two in custody are from the city's economic and technological development zone, the report said.
The explosion is one of the worst recent industrial accidents in the country. Six of those killed were firefighters.
Earlier reports said Sinopec Chairman Fu Chengyu had apologized for the accident and promised to cooperate in the investigation to determine the cause of the blast. Crude oil had been reported leaking from an underground pipeline prior to the explosion.
Chinese President Xi Jinping visited the injured people at hospitals, expressing concerns about safety, China Daily said.