Beijing, China (CNN) -- Four suspects have been detained in a China high-rise fire that killed 53 people, state media said Tuesday.
Monday's fire has hospitalized at least 70 other people, including 17 who were seriously injured, officials said.
A preliminary investigation indicated that unqualified welders caused the fire, the Xinhua news agency said. The welders violated safety rules, Xinhua said.
The 28-story building was under renovation when it went up in flames.
Witnesses said a scaffolding caught fire, and flames then spread to the building.
"The fire became so big because the scaffolding contained a great amount of flammable materials," Zhu Zhixiang, a Shanghai firefighting official told state broadcaster CCTV. "Heavy wind at the time was another factor."
Firefighters rescued more than 100 residents from the burning building. The building houses 150 families, the news agency said.
"I can smell the smoke and police have pushed onlookers farther away," said Peijin Chen, a freelance photographer who lives across the street from the building. "I saw people taken out and put on stretchers."
CNN's Steven Jiang contributed to this report.