An error in calculating the weight of steel beams likely caused a fatal accident at a construction site near JR Tokyo Station last year, an investigative source said Sunday.

Two workers died and three were injured on Sept. 19 when steel beams fell on the seventh floor of a building being constructed by Obayashi Corp., which informed police of the calculation error.

Police are investigating the accident on suspicion of professional negligence resulting in injury and death, as the error may have resulted in an excessive weight load on a temporary foundation supporting the beams, the source said.

File photo taken in September 2023, shows the site of a deadly construction accident in Tokyo's Yaesu area. (Kyodo)

Five beams, each 13 to 18 meters long, weighing a total of about 48 tons fell, Obayashi initially said.

A joint venture of Obayashi and Taisei Corp. has been constructing the 51-floor building with shops, medical facilities and a bus terminal at the site located in a business district about 350 meters southeast of JR Tokyo Station.

Obayashi said on its website in January that responsibility for the accident lies with the company not the workers involved.

It also said construction resumed after reviewing the strength calculation for the support structure and implementing measures to prevent steel beams from falling.

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2 dead, 3 injured after steel beams fall at Tokyo construction site