BEIJING—The death toll from a fire at a Beijing hospital rose to 29, including 26 patients, authorities said Wednesday, and a dozen people had been detained including the hospital’s head and her deputy.

The fire broke out at the private Changfeng Hospital on Tuesday afternoon and forced dozens of people to evacuate. Some who were trapped had to escape from windows using bedsheets tied together.

A nurse, a medical assistant and a family member also died in the incident, said Li Zongrong, deputy head of Fengtai district.

The actual number of casualties from such event may be much higher. The actual number of casualties is difficult to verify, as the Chinese regime routinely suppresses or alters information.

The cause of the fire is under investigation but officials say they believe it originated from welding sparks from work being done in the hospital’s inpatient wing. Among the 12 people who were detained was the head of the construction crew.

A total of 39 people are being treated for injuries, three of them in critical condition, officials said.