The Securities and Exchange Commission had been informally inquiring into the initial public offering of the company, which gained listings on New York and Hong Kong exchanges in 2003.

China Life raised $3.4 billion in what was last year's biggest IPO and, with allegations of hidden accounting irregularities, one of its most controversial.

Now that the SEC is formally investigating, it can compel witnesses to testify and hand over documents.

Credit Suisse First Boston, Citigroup and Deutsche Bank, the IPO's international underwriters, declined to comment.