Congressional investigators say two of every three U.S. corporations fail to report some of their incomes, resulting in tax avoidance of nearly $30 billion.

The General Accounting Office, an arm of Congress, found that tax compliance by corporations has fallen dramatically over the past few years and that the gap between what they owe and what they pay voluntarily has increased by nearly 200 percent.GAO officials were reporting the findings Wednesday to the Senate Permanent Subcommittee on Investigations, which released an outline of the report in advance.