Three of the nation’s intelligence agencies received subpoenas Wednesday afternoon issued by the House Intelligence Committee, Fox News has confirmed, with each of the three demands for documents explicitly naming three top officials of the Obama administration: Susan Rice, who served as President Obama’s White House national security adviser; former CIA Director John Brennan; and former U.N. ambassador Samantha Power.

The three subpoenas, among a total of seven signed by panel chairman Rep. Devin Nunes (R-CA), were served on the Federal Bureau of Investigation, the Central Intelligence Agency, and the National Security Agency, and all three explicitly referenced “unmasking” – a signal that the House panel is intensifying its investigation into allegations that Obama-era aides improperly demanded the “unmasking” of names of associates of President Trump that had appeared, in coded form, in classified intelligence reports, then leaked the data to news media organizations.

The other four subpoenas were issued at the behest of the committee’s ranking Democrat, Rep. Adam Schiff (D-CA), and were said to be duplicative of subpoenas already issued by the Senate Intelligence Committee, which is conducting a parallel probe. These four are focused, sources said, on persistent – but as yet unsubstantiated – allegations of collusion between the Trump campaign and the Russian government, as well as the case of Michael Flynn. The former White House national security adviser was dismissed after three weeks on the job because the White House concluded he had misled Vice President Pence about private conversations Flynn had had with the Russian ambassador late last year.

The other target of these four subpoenas is said to be Michael Cohen, a longtime Trump attorney. Cohen has denied participating in any effort at collusion with the Kremlin. Flynn, through attorneys, has unsuccessfully sought immunity from prosecution in exchange for congressional testimony.

The issuance of the seven subpoenas was first reported by the Wall Street Journal.

The inclusion of Power’s name on the subpoenas marks the first appearance of the former U.N. ambassador in the controversy surrounding the Obama administration’s use of unmasking. Capitol Hill sources told Fox News they are devoting increasing scrutiny to Power – a former historian and winner of the Pulitzer Prize who worked as a foreign policy adviser in the Senate office of Barack Obama before joining his administration – because they have come to see her role in the unmasking as larger than previously known, and eclipsing those of the other former officials named.

Rice has previously denied any improper activity in her use of unmasking. “The allegation is somehow Obama administration officials utilized intelligence for political purposes, that's absolutely false,” Rice told MSNBC on April 4. President Trump said at that time that he personally believed Rice had committed a crime. None of those named on the subpoenas has been formally accused of wrongdoing.

Inquiries placed with representatives of Power and Brennan were not immediately returned.

That Nunes signed the seven subpoenas, as is standard practice, underscored the chairman’s continuing influence over key aspects of over his committee’s probe, despite the fact that Nunes in early April “stepped aside” from his panel’s Russia probe. He insists his decision was not a formal recusal, and he is still awaiting a hearing by the House Ethics Committee, which agreed at the time to investigate whether Nunes had improperly shared classified data with the White House before presenting it to Schiff and the rest of the intelligence committee.

Nunes told Fox News in an exclusive interview on May 19 that he is an active chairman, including continuing to preside over the unmasking angle of the investigation

Investigative sources on the committee’s Republican majority staff told Fox News that the unmasking subpoenas do not reflect a “fishing expedition,” but were issued because documentary evidence already in hand warranted demands for additional documents relating to Rice, Brennan and Power.

Where NSA had previously complied with the House panel’s investigators, sources said that cooperation had ground to a complete halt, and that the other agencies – FBI and CIA – had never substantively cooperated with document requests at all. The investigators believe that even rudimentary document production as a result of the subpoenas will enable them to piece together a timeline linking the unmasking activity to news media reports, based on leaks, that conveyed the same information provided to the officials requesting unmasking.

President Trump and the White House have dismissed the long-running allegations of collusion between Russia and the Trump campaign, and possibly the transition team, as “fake news,” a scandal ginned up by supporters of President Obama and Hillary Clinton to explain the Democratic nominee’s stunning loss to Mr. Trump last November.

However, the Trump administration belatedly acquiesced in the appointment of former FBI Director Robert S. Mueller III as a special counsel to investigate the allegations “and related matters.” Critics of the administration have also pointed to sustained reporting alleging undisclosed contacts between key Trump aides and various Russians – Attorney General Jeff Sessions recused himself from the probe at an early stage because of such contacts – and to a memorandum prepared in February by former FBI director James Comey, leaked a few days after his termination by President Trump, in which Comey alleged that the president had personally importuned him to abandon the FBI’s probe of Flynn.