The FBI is facing pressure to offer additional details about its decision to launch a new investigation into Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE’s private email server.

Director James Comey’s letter to Congress notifying lawmakers about “emails that appear to be pertinent to the investigation” appeared too vague for an announcement sure to shake up the presidential race with less than two weeks until Election Day, critics on both sides of the aisle said.

“Director Comey should give a more complete explanation,” John Weaver, an aide to Ohio Gov. John Kasich (R), said on Twitter. "Too much at stake."

Director Comey should give a more complete explanation. Is this reviewing newly found emails? Is this reopening? Too much at stake. — John Weaver (@JWGOP) October 28, 2016

John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (Texas), the No. 2 Senate Republican, appeared to echo the criticism by retweeting an NBC reporter’s question on whether the "public deserve more" and pressing the issue himself.

Why is FBI doing this just 11 days before the election? — JohnCornyn (@JohnCornyn) October 28, 2016

Supporters of Clinton, such as New York Times columnist Paul Krugman, accused Comey of deliberately jumping into the political fray.

Comey needs to provide full info immediately. Otherwise he has clearly made a partisan intervention, betraying his office. — Paul Krugman (@paulkrugman) October 28, 2016

Comey’s letter to several congressional committees on Friday afternoon sent a shockwave through the political world, reopening a vulnerability that Clinton’s presidential campaign has only barely managed to put at bay.

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The substance of the new emails appears unclear, as do details of the FBI’s action.

Federal investigators had previously determined that, although Clinton was “extremely careless” with her private email setup, she did not intentionally try to circumvent laws protecting sensitive information and so did not break the law. It’s unclear how the discovery of additional emails might affect that determination.

In his letter on Friday, Comey wrote that the FBI “cannot yet assess whether or not this material may be significant” and refused to describe how long the additional work might take.

However, it is sure to loom over the remaining 11 days before Election Day, offering Republican nominee Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE ammunition to hammer the former secretary of State.

Comey had previously been the target of criticism from both sides this summer, when he held a news conference outlining the FBI’s decision not to recommend charges against Clinton or her aides and then detailed the thinking repeatedly on Capitol Hill.

Critics accused him of betraying a double standard by refusing to charge Clinton, while some legal scholars warned that he had set a dangerous precedent by outlining evidence against a person who was not charged with a crime.