The fire, which gutted a 28-storey high-rise in China's busy commercial hub, was sparked by "unlicensed welding carried out contrary to rules", Xinhua news agency said, without citing a source.
"Four suspects have been detained by public security," said the report. It did not say whether those detained were workers or managers.
The swift steps to assign blame for the fire that swept up the 85-metre-high building showed how worried officials are to ease alarm among residents about the more than four hours it took to put it out.
"We feel that the fire rescue measures and methods weren't fast enough, and secondly they weren't vigorous enough," Du Deyuan, a 66-year-old resident who said he lived on the 26th floor and was out when the fire broke out.
"People live in high-rises, and then you have this burn all the way from low down to the 28th storey, burnt so the whole building is blazing red. What could the people inside do?"
China's rapid urban growth is throwing up vast numbers of new high-rise buildings, and while major fire disasters have been relatively rare compared to other developing countries, safety maintenance can be lacking.
"Putting out fires in high-rise buildings is a problem for fire-fighting internationally," Xinhua cited Chen Fei, chief of fire-fighting in Shanghai, as saying.
"Controlling the blaze was very difficult," he added, noting that trucks with ladders and extensions could not get close.
Police Minister Meng Jianzhu said risks of such fires were rising.
"Now is a period when fire disasters can easily occur, and we have to conscientiously absorb the lessons of this disaster," he told officials in Shanghai, according to the Ministry of Public Security website.
As well as 53 confirmed killed, 70 residents were taken to hospital, including 17 with serious burns, said Xinhua. Last year, 1,076 people were killed and 580 injured in fires in China, according to the Ministry of Public Security, which also controls fire-fighting services. It is common to find fire exits blocked or locked in many Chinese buildings, ostensibly to stop thieves or because the space is being used for storage, and fire extinguishers are not widely available. Meng sought to head off public disquiet about the blaze in Shanghai, a city with an urban population of about 13 million which has just finished hosting an expo intended to showcase it as a modern, global metropolis.
"Quickly smooth people's emotions and defuse conflicts," he told officials. "Get to the bottom of the cause, clarify its nature, determine responsibility and deal with this sternly according to the law."
A department building fire in northeastern province of Jilin earlier this month killed at least 19 people and injured 24.
In early 2009, a hotel being built next to the half-finished, hyper-modern new headquarters of Chinese state television in Beijing was consumed by fire after a fireworks display went wrong. One fireman died. — Reuters
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