SEOUL—Prosecutors on Wednesday raided the offices of the Samsung conglomerate as a scandal surrounding the country’s president and her top advisers crept deeper into the nation’s largest business group.

Prosecutors conducted a daylong sweep of the Samsung headquarters in the Gangnam neighborhood of Seoul. At the same time, prosecutors raided the offices of South Korea’s National Pension Service—the world’s third largest, with about $460 billion in assets, and the largest shareholder in many of South Korea’s biggest companies.

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