The FBI said on Thursday it had begun looking into the illegal access of the tax-return information of about 100,000 people disclosed by the Internal Revenue Service earlier this week.

"The FBI has opened an investigation into the recently reported incident at the Internal Revenue Service and is working with our interagency partners to determine the nature and scope of this matter," a spokesman said.

IRS Commissioner John Koskinen said Tuesday cyber criminals had sought to gain access to personal tax information 200,000 times from February to May, and were successful in about half of those attempts.

The FBI spokesman said individuals alerted to the illegal access of information should report "any suspected instances of identify theft" to the FBI's Crime Complaint Center at the website

On Tuesday, the Senate Finance Committee is scheduled to hear testimony about the incident from Koskinen and J. Russell George, the Treasury Department's inspector general for tax administration.