Some 133 workers at the Internal Revenue Service apparently didn't comply with U.S. tax laws during a two-year period but the agency failed to detect them, a government investigator said Tuesday.

About 44 percent of the cases were workers who filed late returns but didn't owe any taxes, said IRS spokeswoman Michelle Eldridge. More than half the cases have already been reviewed and closed because the facts did not merit further review, she said.

Each year, the IRS identifies employees who don't comply with tax laws and disciplines the worst offenders. From 2004 to 2008, the IRS identified an average of nearly 8,800 employees a year who were not complying with all U.S. tax laws.

Some had filed late returns or paid taxes late, while others didn't pay taxes or didn't pay enough.

J. Russell George, the Treasury inspector general for tax administration, said his office found an additional 69 workers in 2006 and 64 workers in 2007 who were potentially not in compliance. The agency has about 107,000 employees.

"To maintain public confidence in the agency entrusted to administer the nation's tax law system, the IRS must ensure that potential misconduct concerning noncompliance with the tax laws is identified and addressed," George said.

Ensuring that IRS employees comply with the tax law is a top priority for the agency, Eldridge said.

"The IRS has the strictest employee tax compliance rules in the federal government," she said. "Employees who are judged to have willful tax compliance problems are terminated, in addition to other potential sanctions."

Eldridge noted that IRS employees have the lowest tax delinquency rate of any large federal agency.

In 2009, 99,000 civilian federal employees owed $1 billion in delinquent taxes to the U.S. government, according to IRS statistics. That's 3.4 percent of all federal workers.

Among IRS workers, less than 1 percent owed delinquent taxes in 2009, according to agency statistics.