The U.S. Internal Revenue Service believes the theft of about 100,000 taxpayers' personal data from its computer system originated in Russia, CNN reported on Wednesday.
The tax agency's criminal unit is leading an investigation into the cyber attack, in which criminals stole information through an online IRS application over the course of four months, and the Treasury Department's inspector general and the Department of Homeland Security are also looking into the breach, CNN said.
The IRS would not comment on the hackers' identities during a news conference on Tuesday when it informed the public of the thefts.
On Wednesday, the IRS had no immediate comment to the CNN report.
Meanwhile, the Senate Finance Committee, which oversees the IRS, said that it will hold a hearing on Tuesday on the data theft.
IRS Commissioner John Koskinen is scheduled to testify, the committee said, along with J. Russell George, the Treasury Department's inspector general for tax administration.
(Reporting by Lisa Lambert; Editing by Sandra Maler and Eric Walsh)