The U.S. Securities and Exchange Commission has opened about a dozen investigations involving complex bundled financial products, The Wall Street Journal reported on Wednesday.

SEC Chairman Christopher Cox told lawmakers on Tuesday that the agency's enforcement division has "about 12 investigations" involving collateralized debt obligations, or CDOs, and collateralized loan obligations, or CLOs, said the report.

Such bundles of debt or loans, which are sold off in smaller segments, have become very popular in recent years and are core drivers behind the surge in leveraged buyouts.

The report quoted people familiar with the matter as saying that the SEC's enforcement division also has opened a preliminary investigation into the issues surrounding the Bear Stearns hedge funds, which invested in complex financial instruments backed by subprime mortgages.

Earlier this year, the SEC enforcement division formed a subprime working group, which is looking at a range of topics from the securitization process to troubled subprime issuers. One area of concern involves the lack of accurate pricing in the CDO market.