SEOUL, South Korea (AP) — South Korean police raided the headquarters of Starbucks’ local operation as part of an investigation into a controversial marketing campaign that was perceived as defaming 1980s' pro-democracy movements.

Investigators searched the Starbucks headquarters in southern Seoul on Tuesday, police said Wednesday.

Police gave no further details, but South Korean media reported investigators also were sent to the residences of Starbucks executives.

The coffee chain triggered a public uproar when it attempted to promote a series of stainless-steel tumblers it called “SS Tank” by declaring May 18 to be “Tank Day.” The date marks the anniversary of a 1980 pro-democracy uprising in the southern city of Gwangju that was brutally oppressed by South Korea's then military-backed rulers.

The campaign also used the slogan “Thwack it on the table!” which many read as a reference to a notorious 1987 police statement that attempted to cover up the torture death of student activist Park Jong-chol. Authorities had falsely said Park died after investigators “hit the desk with a thwack.”

The public outrage prompted Shinsegae Group, which owns a 67.5% stake in Starbucks Korea, to cancel the campaign within hours. It also fired the chief executive of Starbucks Korea. Shinsegae Chairman Chung Yong-jin later issued a televised apology. Starbucks Korea closed all of its stores nationwide early on June 22 for mandatory history and social sensitivity training.

The police investigation was launched following complaints from relatives of the victims of the Gwangju crackdown, which official records say left about 200 people dead.

South Korea achieved a genuine democracy in 1987 when then authoritarian President Chun Doo-hwan, who led the Gwangju crackdown, was forced to accept a constitutional revision introducing direct presidential elections in the face of massive pro-democracy demonstrations.