Beijing, China (CNN) -- Eight suspects have been detained in a China high-rise fire that killed 53 people, state media said Tuesday.
At least 70 other people were injured, 15 critically, in the Monday fire in Shanghai's Jing'an district, according to the Xinhua news agency.
Another 50 people were missing, mostly senior citizens in their 60s and 70s, according to the news agency.
A preliminary investigation indicated that unqualified welders caused the fire, according to Xinhua. The welders violated safety rules, according to the report.
Some of the welders were among those detained, Xinhua said.
The 28-story building was under renovation when it went up in flames. The blaze broke out on the north side of the 10th floor, according to Xinhua.
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 28-story 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 said at the time of the blaze. "I saw people taken out and put on stretchers."
CNN's Steven Jiang contributed to this report.