With news Thursday that a preliminary criminal investigation has been launched into some of Wall Street’s biggest firms concerning potentially misleading sales of CDOs to clients, the fiduciary train is set to gain additional momentum in Washington. Firms under investigation by federal prosecutors and securities regulators include J.P. Morgan, Citigroup, Deutsche Bank and UBS, according to the lead storyin today’s Wall Street Journal. Meanwhile, the SEC has issued civil subpoenas to these firms, as well, as part of a larger investigation into the banks’ mortgage-related trading and selling practices. Morgan Stanley and Goldman Sachs face similar investigations.

After all, recent hearings in Washington with Goldman Sachs executives helped to bring the fiduciary issue back into play on Capitol Hill. What prompted the investigations is that the firms being probed created CDOs at the behest of clients who wanted to bet against the deals, and some of the firms bet against those deals themselves, while selling them to other clients.

Of course, it is still impossible to predict what final form the fiduciary issue will take in financial services regulatory reform legislation. The House of Representatives passed a bill in December that would give the SEC rulemaking authority to extend the fiduciary standard to brokers when they provide advice to retail clients. The companion bill in the Senate, sponsored by Christopher Dodd (D, Conn.), calls for a one-year study of the fiduciary issue, but a number of competing amendments to the bill have been introduced in the past week that would either echo language in the House bill, or extend a fiduciary standard to any advisor who provides investment advice to retail or institutional clients. These have yet to be debated on the Senate floor.