SEOUL, Nov. 6 — A corruption scandal at the Samsung Group, the South Korean conglomerate, widened Tuesday as prosecutors opened a formal investigation into charges that its chairman masterminded a broad scheme of bribery and illegal transactions.

Prosecutors are investigating three major accusations of criminal behavior: the creation of a slush fund; the bribery of prosecutors and government officials; and an effort by the chairman, Lee Kun Hee, and his aide to illegally help his son take over control of Samsung.

“We are ready to unveil the truth through a stern, fair and thorough probe,” said Kim Kyong Soo, a prosecution spokesman.

In previous scandals that have plagued Samsung, several executives have been convicted of illegally trying to help Mr. Lee’s son, Jae Yong, take control of management, and of bribing politicians. But Mr. Lee’s family has escaped largely unscathed. This has led critics to charge that Samsung runs a vast network of bribery and influence-peddling through the government, the judicial branch and the media, making the Lee family untouchable — a claim vehemently rejected by Samsung.

This time, the group is facing a potent whistle-blower: Kim Yong Chul, its former chief lawyer, who said he was personally involved in bribing and fabricating court evidence on behalf of Mr. Lee and Samsung.

Samsung denied all of Mr. Kim’s allegations Tuesday, saying that he was turning against Samsung out of “personal grudges.”

In a legal complaint filed with prosecutors on Tuesday, Mr. Kim, who worked as an internal lawyer for Samsung for seven years until 2004, said that Mr. Lee and his top aides illegally ordered transactions that allowed his son to acquire Samsung shares from Samsung affiliates at unfairly low prices.

When prosecutors investigated one transaction in 2003, Mr. Kim said lawyers of his legal division at Samsung trained Samsung executives to serve as scapegoats to protect Mr. Lee, even though those executives were not involved. Two of the executives were found guilty in a court ruling in October 2005, and Samsung is appealing.

In interviews with South Korean media in the last few days, Mr. Kim said he was “sidelined” by Samsung after he refused to pay 3 billion won, or $3.3 million, in a bribe to the judge presiding over the case.

Mr. Kim’s accusations took on a new drama on Monday, when he gave a nationally televised news conference in a Catholic church in Seoul.

“Samsung instructed me to commit crimes,” he said at the news conference. “A basic responsibility for all Samsung executives is to do illegal lobbying, buying people with money.”

On Monday Samsung issued a 25-page rebuttal denying all major accounts of Mr. Kim’s allegations. It noted that Mr. Kim did not provide evidence to support his claims.

During his news conference, Mr. Kim did not keep promises he made last week to reveal internal Samsung documents, including lists of prosecutors who he said received bribes. He said that he would do so later.

Two influential civic groups — People’s Solidarity for Participatory Democracy and Lawyers for a Democratic Society — filed a legal complaint Tuesday on behalf of Mr. Kim, prompting the official opening of the investigation by prosecutors.