Iranian authorities have widened an investigation into a bank fraud case believed to be the largest in the country's history.

Iran's State Prosecutor Gholam Hossein Mohseni Ejei told state television that 67 people have been summoned for questioning and 31 detained in the $2.6 billion case.

He said the main suspect, Iranian businessman Amir Mansour Khosravi, has begun confessing to the scam and is sharing details of the case with investigators.

The accusations focus on the alleged use of forged documents to obtain loans from one of Iran's top financial institutions. Iran's Fars news agency reports the loans were used to buy state-owned companies.

On Sunday, Iran's parliament launched an impeachment process against Economy Minister Shamseddin Hosseini, saying his poor monitoring of banks is to blame for the scam.

Parliamentarians also accused him of hiring unqualified people for key positions in his ministry.

Iran's conservative-dominated parliament has maintained pressure on President Mahmoud Ahmadinejad's government, which some parliamentarians have called a “deviant current” that seeks to undermine Supreme Leader Ayatollah Ali Khamenei.