Police stand guard at the accident site in the downtown area of Shanghai, east China, Nov. 16, 2010. The death toll of a big fire that engulfed the high-rise building in downtown Shanghai had risen to 53 by 9:30 a.m. Tuesday, local authorities said. More than 70 people injured are being hospitalized. (Xinhua/Pei Xin)

An initial investigation has blamed the disaster, one of the deadliest in decades, on unlicensed welders, said Zhang Renliang, chief of Jing'an District Government, said at a press conference.

Some of the welders were among those detained, but the identities of the others were not immediately known.

Deputy district chief Li Weiping said 15 of the injured were still critically ill as of 4 p.m..

The blaze erupted on the north side of the 10th floor of the 28-story building, said Chen Fei, director of the city's fire-fighting bureau.

The 85-meter-high building housed more than 156 households with 440 people. Residents moved into the building in March 1998 after construction was completed in 1997.

The building was undergoing an energy-saving renovation project when the fire started after lunchtime.

The Jing'an District Construction Corp. and Shanghai Jiayi Decoration Corp. were the contractors for the project.

The city's emergency response center received a fire report at about 2:15 p.m. and authorities dispatched 122 fire engines and more than 1,300 fire-fighters to the site.

More than 100 residents were rescued and the blaze was extinguished by 6:30 p.m..

Local authorities are still trying to determine how many residents were in the building when the fire broke out and the number of missing people.

Police and fire-fighting authorities have promised to disclose more information about the cause of the blaze as soon as possible.