The prosecutors’ office has decided to launch a criminal investigation against Rep. Lee Sang-deuk. The law-enforcement agency says it will look into the large sum of money of unknown origin that is deposited in the bank account of one of his secretaries. Few believe the investigation will be limited to that individual case, given all the bribery suspicions against him, both old and new.

The investigation will undoubtedly draw great public attention, not only because he is a senior lawmaker but because he is an older brother of President Lee Myung-bak. It may not be a mere coincidence that the agency is pointing its dagger at the throat of the lawmaker when the president’s final year in office has just started. Past cases show that it acted resolutely against those close to the president when the president’s grip on power began to loosen.

President Kim Young-sam saw his son sent to prison in the final year of his five-year presidency. So did President Kim Dae-jung. President Lee’s immediate predecessor, Roh Moo-hyun, who had retired to his country home, killed himself when prosecutors targeted him in a criminal investigation into bribery.

Moreover, prosecutors took no time in starting investigations against opposition politicians when they were suspected of corruption. No wonder they are accused of being afraid of “plucking even a feather” from a criminal suspect with indomitable power and being ruthless against those whose power is waning or lost.

Rep. Lee’s trouble began four years ago when he ran for election to the National Assembly for a sixth term. Foes and friends alike demanded that he retire from politics and do some other work if he wanted his brother to have a successful presidency. They said he could do volunteer work, engage in nonprofit educational projects or take up the post of diplomatic mission chief in a remote country. But the advice fell on deaf ears.

It was not long before he was accused of being inappropriately involved in the administration’s personnel management, advocating for those close to him to get promotion or appointment to high public posts. He was also criticized for throwing his weight behind his favored fellow lawmakers when they were pursuing high posts in the ruling party or in the legislature.

His influence peddling antagonized so many in the party that Lee the politician had to make an oxymoronic promise in June 2009 that he would no longer involve himself in political issues. Instead, he said he would devote himself to what he called resources diplomacy ― diplomacy aimed at securing the steady supplies of key resources from abroad.

Rumors recently had it that Lee took 200 million won from a person who had been pursuing the ruling party’s nomination for election to the National Assembly in 2008. They were followed by a scandal involving one of Lee’s aides, who allegedly took 600 million won in bribes from a businessman. Prosecutors investigating the case found 700 million won of suspicious origin deposited in another secretary’s account.

Lee told the prosecutors that the money came from his private coffers he was keeping at home, adding that it was part of the leftovers of the proceeds from a property sale and gift money he had received when one of his children was married. Apparently unconvinced by this explanation, the prosecutors decided to open a formal investigation into the case.

Now comes a new bribery allegation against him. According to news reports, prosecutors are in the process of confirming the intelligence that he took hundreds of millions of won from an insolvent savings bank whose operations have been suspended. He denied the allegation.

True, it is debatable why it had taken so much time until the prosecutors’ office decided to start a formal criminal investigation against Lee. But this question may well be shelved until after the investigation is complete.

The prosecutors will have to look into all the allegations and leave no stone unturned in its inquiries. Were he ever cleared of all suspicions, he should still be held accountable for the criminal offense of failing to report the 700 million won to the electoral watchdog.