By Ellen Nakashima, Sari Horwitz and Matt Zapotosky, Washington Post–

From the moment they secured a warrant, dozens of FBI agents worked night and day to analyze a trove of messages that they thought might help advance their probe of Hillary Clinton’s use of a private email server, according to a U.S. official.

The pressure was intense. FBI Director James B. Comey had told legislators in late October — less than two weeks before the election — that the bureau’s work had resumed, igniting a firestorm of criticism that his revelation had affected the election. The agents’ work, at first, seemed endless. They had to use special software to sift through some 650,000 emails.

But on Sunday, just two days before the election, Comey announced that the team had news to share. After reviewing “all of the communications that were to or from Hillary Clinton while she was Secretary of State,” he wrote, investigators had “not changed our conclusions.”

The messages, U.S. officials familiar with the case said, were either personal or duplicative of those found earlier in the investigation.

The recommendation marked the culmination of a nightmarish 10 days for the bureau. But coming two days before the election, it also generated renewed skepticism from both political parties about the FBI’s handling of the high-profile case.

Republicans said the announcement was vague and that they had unanswered questions about how investigators concluded that Clinton should face no charges in the first place. Democrats, meanwhile, said they remained concerned that Comey had told legislators so close to Election Day that the email investigation was resuming, though they were heartened to see the matter ostensibly put to rest.

“In the days that come,” said Rep. John Conyers Jr. (D-Mich), “we will have many questions about the handling of this investigation.”

From the moment Comey announced in July that he was recommending Clinton not be charged, the bureau has been under pressure and Comey at the center of a political firestorm.

Republicans have questioned whether FBI agents were thorough and aggressive enough, while Democrats have said that Comey was wrong to offer so much public detail on his findings. When he made the July announcement, Comey opined that Clinton and her staffers were “extremely careless” in their handling of classified information — an allegation that critics say maligned the Democratic presidential nominee without a trial in which she could defend herself.

Comey has said he offered so much detail to assure the public that the investigation was handled appropriately and without political influence. He testified for hours before Congress about what investigators had found and released to the FBI’s website hundreds of pages of interview summaries and other documents in the case.

But with his public path came pitfalls. And when FBI agents were looking into allegations about disgraced former congressman Anthony Weiner (D-N.Y.) and found on Weiner’s computer emails potentially relevant to the Clinton probe, Comey had a dilemma.

Should he update Congress, with the election so close, that the Clinton email investigation had resumed? Or, with so little information, should he wait, potentially inviting criticism that he had buried information to help Clinton get elected?

After a full briefing on Oct. 27, Comey chose the former, sending a short letter to legislators the next day saying that the FBI had “learned of the existence of emails that appear to be pertinent to the investigation” and that agents would “take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation.”

Weiner is the estranged husband of top Clinton aide Huma Abedin, and U.S. officials have said that they found correspondence of Abedin and Clinton on his computer. Abedin has said that she is unsure how her emails would have ended up on her husband’s laptop. When agents first found the materials, they were legally prohibited from sifting through them for Clinton email probe purposes. They got a warrant to do so Oct. 30.

“I don’t think anyone was unaware of the calendar,” one senior U.S. law enforcement official familiar with the probe said.

As the agents worked, the bureau faced a seemingly endless torrent of criticism. News leaked that agents in the FBI’s New York field office had been advocating for a separate investigation of the Clinton Foundation, even though public integrity prosecutors had told them that they did not have a case. That fueled the perception that at least some in the bureau, an organization of predominantly white men, might have partisan motivations, and Democrats called for the Justice Department inspector general to look into the matter.

Republicans also questioned whether — if the Clinton email investigators had used more aggressive tactics — they might have uncovered information that could have been used by those wanting to look into the Clinton Foundation.

Given the reasoning Comey laid out for not recommending charges in July, it was always unlikely that the new email review would change anyone’s mind. The FBI director already had said that classified information traversed Clinton’s private server. He said that in 110 emails, information was deemed sensitive enough to be classified at the time it was sent or received. Eight chains, he said, even included information deemed “top secret” — the highest level of classification. Adding to those totals would not seem a reason to alter his conclusion.

Now, though, the bureau will face more questions. In a statement, Sen. Charles E. Grassley (R-Iowa), the chair of the Judiciary Committee, said that the “vague” announcement by the FBI failed to provide context and that he was unsure if the review was over.

“The growing number of unanswered questions demand explanations: Is the FBI continuing to review the newly-revealed emails?” Grassley said. “Did the FBI limit its review to email from when Clinton was Secretary of State, leaving out emails that could shed light on possible obstruction of Congress?”

This time, though, Comey’s timing did not seem as much of an issue. Republicans and Democrats had called on him to complete the new review swiftly and announce his findings publicly. Ron Hosko, a former assistant director at the FBI, said that he believed Comey gave Congress information when it was available to him and that Sunday’s revelation served as evidence of that.

“I thought, if anything, now would be the time to sit quietly until after the election, but here we are again,” Hosko said. “They were able to give clarity on a Sunday evening, two days before the election, and he felt like, ‘If we have the question answered now, we have to answer the question now.’ ”