The six-page letter, signed by Senate Finance Committee Chairman Charles Grassley (R-Iowa) and Sen. Max Baucus (D-MT), was sent last week to 10 tax preparation firms. The companies were not identified for the press.

According to WSJ, the letter said that a government report published last month, which outlined errors and even cheating by national tax-prep firms, raised "serious concerns" about the accuracy and quality of tax returns by tax preparers at these firms. In addition to operations, the letter also asked for the firm's policy on the use of or disclosure of clients' tax-return information.

The senators are sponsors of a bill that would require examinations for and impose continuing education requirements on any person who prepares federal income tax returns, and would give the IRS authority to regulate all paid income tax preparers.

There are more than 1 million paid tax preparers nationwide -- as many as 1.6 million by some estimates. While a good portion of this group includes CPAs, enrolled agents, and other highly trained professionals who are regulated, it also includes preparers who have little training and who face little or no regulation.