The Seosomun overpass in Seodaemun District, western Seoul, is seen on May 28 after it partially collapse during demolition work on May 26. [JANG JIN-YOUNG]

Seoul city officials visited the Seosomun Overpass demolition site and issued a safety warning three months before a section of the structure came down and killed three people, according to a document obtained by a lawmaker on Thursday.

The warning had stressed the need for strict on-site management to prevent serious industrial accidents such as falls and workers being crushed.

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On Tuesday afternoon, a section of Seosomun Overpass collapsed during demolition work. The overpass, which was built in 1966, was being dismantled due to safety concerns. Demolition began on Aug. 17 last year and was scheduled for completion in early June.