Police in South Korea have searched the headquarters of Starbucks Korea as part of an investigation into a marketing campaign that drew widespread criticism earlier this year.

According to Reuters, the state broadcaster KBS reported that investigators were seeking documents and other evidence not included in materials previously submitted by Starbucks Korea’s parent company, Shinsegae Group.

The probe relates to allegations that the campaign referenced the 1980 military crackdown on pro-democracy protesters in Gwangju, triggering public backlash over its perceived insensitivity.

Starbucks Korea said in May that the controversy caused a “very significant” decline in sales.

The fallout also weighed on Shinsegae, with its shares falling after chairman Chung Yong-jin issued a public apology over the campaign. The retailer also dismissed Starbucks Korea chief Sohn Jeong-hyun, saying he was responsible for the “inappropriate marketing”.

Starbucks Korea and Shinsegae told Reuters they viewed the search as part of the ongoing investigation into the company.