South Korean investigators visited Apple's HQ earlier this week, and it's been reported that the authorities conducted a raid in Apple's offices in the country.

Investigators are said to have questioned Apple regarding its business affairs, with the grilling coming just a day before the launch of Apple iPhone X in South Korea.

Officials have refused to give any explanation for the raid. According to Metro, the raid was part of an ongoing probe into "unfair" contracts between Apple and South Korean firms.

Officials were told to ask Apple employees about its business practices. It is still not clear whether the raid concerned only questioning or included the collection of documents and other shreds of evidence as a part of the probe.

Apple products are wildly popular in South Korea, which is home to several tech giants, including Samsung and LG.

Senior Vice President of worldwide marketing Philip Schiller speaks during a media event at Apple's new headquarters in Cupertino, California, on September 12, 2017.JOSH EDELSON/AFP/Getty Images

Apple and the Korean Fair Trade Commission (FTC) have had a long-standing tussle. Last year, when Apple grabbed a historic 33 percent share of the South Korean smartphone market, the FTC launched a task force dedicated to exploring whether foreign firms were hurting the domestic smartphone market.

Last year, when the raid occurred in the Apple headquarters in South Korea, the FTC investigators looked at the contract terms Apple required carriers to accept in order to sell iPhones.

These included provisions for carriers to buy a minimum number of iPhones, sharing the burden of repair costs, and the inability of repair firms to file lawsuits against Apple Korea for a year after a dispute.

Endpoint Technologies Associates president Roger Kay accused the commission of having a "protectionist agenda," writing in Forbes in 2015 that the agency had "pretty much run amok in recent years, slapping spurious charges on foreign companies."

In recent years South Korea's government has kept a keen eye on possible cases of corruption. In August, Samsung Group head Jay Y Lee was sentenced to 5 years in prison after a six-month trial ended in his conviction for bribery and other charges that led to the downfall of former South Korean President Park Geun-Hye.