Seoul Metropolitan Police Agency officials raid the office of a contractor involved in the deadly Seosomun overpass collapse in Seongdong-gu, Seoul, on Friday. (Yonhap) Police raided Seoul city offices and construction companies Friday as authorities widened their investigation into the overpass collapse that killed three workers earlier this week.The Seoul Metropolitan Police Agency began searches at 9 a.m. at seven locations, including the Seoul Metropolitan Government’s Urban Infrastructure Headquarters, which commissioned the demolition; the project’s main contractor Heunghwa Construction; the supervising firm, Suseong Engineering; and subcontractors and field offices near the demolition site, police said.Police said they would "strictly investigate the case by closely analyzing materials secured through the searches to clearly determine the cause of the accident and where responsibility lies."The raids came three days after part of the Seosomun overpass in western Seoul collapsed during demolition work on Tuesday.Work had reportedly been halted after abnormal signs were detected at the site, and a safety inspection was underway when part of the concrete slab gave way, killing three people.Police and the Ministry of Employment and Labor are seeking structural design drawings, safety management plans and other records related to the demolition.The ministry, which has formed a separate investigation team, plans to examine whether the work was carried out in line with approved design documents and safety procedures.Investigators also secured materials from the contractor’s on-site office and storage facilities. Police had earlier conducted a site inspection with the National Forensic Service shortly after the accident.The Seoul Metropolitan Government said it had submitted demolition-related documents, including safety management plans and closed-circuit television footage showing the site at the time of the collapse.“The search of the Urban Infrastructure Headquarters is understood as part of the investigation to determine the cause of the collapse,” the city said in a statement. “As the project owner, Seoul City is fully cooperating with the investigation by submitting relevant materials.”