South Korean police have raided Starbucks Korea’s headquarters as part of a criminal investigation into the company’s controversial “Tank Day” promotional campaign, which was accused of mocking one of the country’s darkest chapters.

More than 40 investigators from the Seoul Metropolitan Police Agency’s Public Crime Investigation Unit searched the company’s headquarters in Seoul’s Gangnam district on Wednesday morning, seizing internal documents and digital evidence linked to the controversial “Tank Day” campaign, according to Yonhap.

The warrant was issued on suspicion of insult, with investigators trying to establish whether Starbucks Korea employees deliberately approved marketing that referenced the 18 May Gwangju uprising and another defining moment in South Korea’s struggle for democracy.

The controversy began when Starbucks Korea launched a promotion for its “Tank” range of tumblers under the banner “Tank Day”. The date coincided with the anniversary of the 1980 Gwangju uprising, when troops sent by the military dictatorship brutally crushed pro-democracy protests in the southwestern city.

Official figures put the death toll at around 200 people, although victims’ organisations and historians have long argued that hundreds more were killed.

For many South Koreans, the campaign immediately appeared to invoke the military tanks associated with the suppression of demonstrators.

Further criticism was levelled against the campaign’s accompanying slogan, “Tak! on the desk”, which many interpreted as a reference to the dictatorship’s notorious attempt to conceal the torture death of student activist Park Jong-cheol in 1987.

At the time, authorities falsely claimed Park had died after an officer struck a desk, making a “tak” sound.

The explanation later became synonymous with the military regime’s brutality and deception after it emerged that Park had died under torture. His death helped galvanise the nationwide June Democracy Movement, which forced the military government to accept direct presidential elections later that year.

The campaign was withdrawn within hours as outrage spread across South Korean social media. Calls for a boycott quickly followed, with users posting photographs and videos of Starbucks cups being crushed or thrown away, while the phrase “tal-buck” – meaning to quit Starbucks – trended online.

Shinsegae Group, which operates Starbucks Korea under licence and owns a controlling stake through its affiliate E-Mart, dismissed Starbucks Korea chief executive Sohn Jeong Hyun and launched an internal investigation into how the campaign had been approved.

Starbucks Korea apologised, saying it recognised that “wording connected to the 18 May Gwangju Democratic Uprising, which carries profound historical significance, was used in an extremely inappropriate manner”.

“We once again apologise for causing this controversy on a memorial day that honours the noble spirit of the May 18 democratic movement and the sacrifices of its victims,” the company said, adding that it would strengthen internal review procedures and introduce training on historical awareness and ethical standards.

Shinsegae Group chair, Chung Yong Jin, also apologised, describing the promotion as “an inexcusable wrongdoing that trivialised the pain and sacrifice of everyone who devoted themselves to advancing democracy in this nation”.

Starbucks Corp, the Seattle-based coffee chain, issued a separate apology, saying: “While unintentional, this should never have happened. We recognise the deep pain and offence this has caused, particularly to those who honour the victims, their families and all who contributed to Korea’s democratisation.”

Multiple complaints were filed by organisations that represented people who had been recognised for their role in the May 18 Democratic Movement, which led to the criminal investigation.

The complaints also alleged violations of South Korea’s Special Act on the May 18 Democratisation Movement, which protects the historical record of the 1980 uprising, although prosecutors later concluded that only the alleged offence of insult supported the search warrant.

Police first sought a search warrant in June but prosecutors returned the application, arguing that investigators should continue with voluntary inquiries because Starbucks Korea was cooperating with the investigation, according to the Korea Herald.

Investigators later concluded that Shinsegae’s internal audit had failed to fully establish how the campaign had been conceived. According to the company, only two of the five employees directly involved in planning the promotion voluntarily handed over their mobile phones for forensic examination.

Three others, including an executive, refused, with Shinsegae saying it had no legal authority to compel them to surrender the devices.

South Korean president Lee Jae Myung had previously condemned the campaign, calling it an affront to the memory of those who fought for democracy. Following Wednesday’s raid, the opposition People Power Party accused police of acting on behalf of the government.

Writing on Facebook, party leader Jang Dong Hyeok said the search showed police had become “the regime’s loyal hounds”, while lawmaker Joo Jin Woo described it as “the opening act of a Chinese-style police state” and accused officers of “pandering to power”, according to Chosun Daily.

A Shinsegae Group spokesperson confirmed to AFP that police had raided the company’s headquarters “as part of the ongoing investigation”, but declined to provide further details.