Special Counsel Robert Mueller has tapped multiple grand juries, including juries in Washington and Virginia, in an effort to gather evidence in the ongoing federal investigation into Russia's meddling in the U.S. presidential election, three sources familiar with the matter told NBC News.

The Wall Street Journal reported Thursday that Mueller had impanelled a separate grand jury in Washington, but sources familiar with the matter say that Mueller is using existing grand juries in both Washington and Virginia.

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Mueller was appointed the special counsel by the Justice Department in May to investigate links between President Donald Trump's presidential campaign and Russia. Mueller led the FBI for 12 years under presidents George W. Bush and Barack Obama. He is second only to J. Edgar Hoover for longest tenure for an FBI chief.

The use of a grand jury is part of the normal process of a complex investigation and it is a tool not just for hearing testimony but for learning about and obtaining records from individuals or companies. It is a powerful tool prosecutors can use to seek indictments and subpoenas, and may be a signal Mueller's work is far from over.

Related: Flynn, Manafort Are Key Figures In Mueller Russia Investigation

As NBC News has previously reported, the FBI, with the help of the Treasury Department, the CIA and other agencies, is examining evidence of possible contacts, money transfers and business relationships between a variety of Trump associates and Russian officials, the sources say

Records related to business transactions for former campaign chairman Paul Manafort and former National Security Advisor Michael Flynn are among the documents that have been subpoenaed through the grand jury process.

Related: Trump Mocks Investigations: 'Are There Any Russians Here Tonight?'

Law enforcement officials tell NBC News that both Flynn and Manafort are formally considered "subjects" of a criminal investigation, though their lawyers say they have done nothing wrong. A "subject" is someone whose conduct is being examined by the investigation, and may be suspected of a crime. So far, no public evidence has surfaced linking Flynn and Manafort to the Russian interference effort.