The explosion in Jiangsu province's Yancheng city last month was one of the worst industrial accidents in the country in recent years and led to the closure of the plant.
Six employees from the Jiangsu Tianjiayi Chemical company, whose plant was involved in the blast, have "major responsibilities" in relation to the incident, the Yancheng government said Saturday in a brief statement on its official Twitter-like Weibo account.
It said that "criminal coercive measures" have been taken against them -- a vague term which indicates detention, arrest or being put under temporary house arrest until police complete investigations, according to an explanation on the Chinese parliament's website.
Three others had also earlier been detained in connection with the blast.
The March 21 explosion razed an industrial park and blew out the windows of surrounding homes.
The company, with 195 employees, was established in 2007 and mainly produces raw chemical materials including anisole, a highly flammable compound.
It has a history of violating environmental regulations, according to online records from Yancheng city's environment and ecology bureau.
In 2015 and 2017, the firm was fined for breaking rules on solid and water waste management.
Deadly industrial accidents are common in China, where safety regulations are often poorly enforced.