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FBI opens investigation into data breach at IRS

The FBI has opened an investigation into the recent data breach at the Internal Revenue Service, CNBC has learned Thursday. (Tweet this)

The FBI said that people contacted by the IRS should take the necessary steps to monitor and safeguard their online presence and information. Any suspicious activity should reported to the FBI at www.ic3.gov.

Read MoreIRS believes thieves who stole tax data from 100K were from Russia: AP, citing sources

The investigation by the FBI indicates the breach was far more serious than previously thought. Earlier this week the IRS announced that about 100,000 people had their data, and possible their tax returns, hacked on the IRS website.

The FBI's probe will supplement a current investigation by the IRS criminal division, and add more resources in what could be a fight against global hacker gangs.

Read MoreIRS breach: What taxpayers should do now

—CNBC's Eamon Javers contributed to this report.