A federal grand jury has subpoenaed three former CIA and FBI officials who were involved in the federal government's investigations into alleged Russian meddling to assist President Trump's 2016 campaign, a source familiar with the matter told CBS News.

The subpoenas went out to former CIA Director John Brennan, former FBI counterintelligence official Peter Strzok and former FBI attorney Lisa Page, according to the source, who added that more subpoenas could be issued in the coming days.

The source said the investigation focuses on the Trump-Russia matter — an issue that has rankled Mr. Trump and his allies for years.

CBS News has reached out to the CIA, the Justice Department and Brennan, and attorneys for Strzok and Page.

Fox News Digital was first to report on Brennan, Strzok and

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