It is unusual, if not rare, for the prosecution to conduct a search and seizure at a government agency on a court-issued warrant. But it did so when it raided the Seoul regional office of the National Tax Service on Thursday. The next day, it also served a search and seizure warrant at the Korea Exchange.

The searches and seizures are part of a wide-ranging investigation into a case involving CJ Group, which is suspected of creating slush funds, hiding wealth in tax havens, trading stocks on insider information, evading taxes and committing other crimes. The prosecution is also planning to seek international judicial cooperation in its investigation into CJ Group, which is suspected of operating shell companies in Hong Kong and the British Virgin Islands.

The prosecution took tax records filed by CJ Group chairman Lee Jae-hyun and his sister and vice chairwoman, Lee Mie-kyung, from the Seoul regional office. At the Korea Exchange, it took documents on the trading of shares of CJ and CJ Cheiljedang to look into allegations of their illegal trading through offshore accounts.

But CJ Group cannot be the sole target of international judicial cooperation, now that hundreds of Koreans are reportedly hiding assets in the Virgin Islands, the Cayman Islands and other tax havens. According to a recent disclosure by the Korea Center for Investigative Journalism, those include Lee Soo-young, chairman of the industrial materials manufacturer OCI, and his wife, as well as a former Korean Air president.

Sooner or later, the prosecution will have to start new investigations into the cases involving those businessmen and others suspected of offshore-driven tax evasion. Moreover, the Korea Center for Investigative Journalism, which is working with the International Consortium of Investigative Journalists, has promised to disclose more findings when corroboration is completed next month.

The case involving CJ Group dates back to 2008, when one of its finance officers was accused of hiring a hit man to kill a private moneylender. The police found that its chairman was managing 320 billion won in 500 bank accounts held in the names of group executives. Closing the investigation, the police concluded Lee dodged tax payments and requested the tax office to press criminal charges against him.

But the tax office ignored the request and, instead, did nothing but levy 170 billion won in inheritance tax. Suspicions about his tax dodging were raised again in a separate case the next year. Yet, the prosecution took no action against him.

What the prosecution needs to do is not limited to determining whether or not the Lee family breached the law in filing tax returns and trading shares. It also will have to shed light on why the tax office did not press criminal charges against the CJ chairman in 2008 and why it did not delve into the suspicions in 2009. Was there a powerful figure behind him?