Two Qingdao city employees also were in police custody, local authorities said a statement late Monday.
The Sinopec pipeline, aligned along the city's drainage network, ruptured early Friday and spilled oil along a long stretch of road, as well as into the nearby sea. Hours later the oil exploded as workers tried to clean up the spill.
China's government demanded accountability in the disaster, the deadliest involving Sinopec. Yang Dongliang, head of the State Administration of Work Safety, called it a "very serious" accident caused by irresponsibility.
Yang said the pipeline's layout was unreasonable, that its safety management was lax and that Sinopec and local officials failed to take enough preventative measures after the leak, such as cordoning off the area and evacuating residents, according to a statement posted on the administration's web site.
Three more bodies were found on the scene Monday, raising the death toll to 55, Qingdao police said. They said of the 52 confirmed dead earlier, 42 were male. Friday's blast also left 136 people injured and nine others missing.
Sinopec's expansion of petrochemical projects has met with resistance from members of the public, and Friday's blasts have added to growing concern about safety and environmental risks. Sinopec apologized Saturday for the explosions. It said it would investigate and "give timely reports."