FBI agents investigating Hillary Clinton's use of a private email server while Secretary of State knew early last month that messages recovered in a separate probe might be relevant to their case, but waited weeks before briefing the FBI director, according to people familiar with the case.

Director James Comey has written that he was informed of the development on Friday, and he sent a letter to legislators the next day letting them know that he thought the team should take "appropriate investigative steps designed to allow investigators to review these emails".

That missive ignited a political firestorm less than two weeks before the election. Almost instantly, Comey came under intense criticism for his timing and for bucking the Justice Department's guidance not to tell Congress about the development. And his announcement means that Clinton could have to contend with the news that the FBI has resumed its investigation of her use of a private email server - without any clarity on whether its investigators will find anything significant - up to and beyond Election Day.

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The FBI has obtained a warrant to search the emails found on a computer used by former congressman Anthony Weiner that may contain evidence relevant to the investigation into Clinton's private email server, according to law enforcement officials. The warrant was obtained in New York, as FBI agents there have possession of the laptop.

One official said the total number of emails recovered in the Weiner investigation is close to 650,000 - though that reflects many emails that are not relevant to the Clinton investigation. However, officials familiar with the case said the messages include a significant amount of correspondence associated with Clinton and her top aide Huma Abedin, Weiner's estranged wife.

People familiar with the case said that agents on the Clinton email team had known about the messages since soon after New York FBI agents seized a computer related to their investigation into Weiner, who has been accused of exchanging explicit messages with a 15-year-old girl.

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Officials said the agents probing Clinton's private email server didn't tell the director immediately because they were trying to better assess what they had. "It's a step-by-step process," said one senior law enforcement official. "There are many steps along the way that get you to a place where the director can be appropriately briefed in order to make a decision" about whether to move forward.

Investigators will now look at whether the newly uncovered emails contain classified information or other evidence that could help advance the Clinton email probe. It is possible, though, that the messages could be duplicative of others already recovered elsewhere or that they could be a collection of benign, personal notes.

Several law enforcement officials with technical expertise said it is generally not difficult to create software to analyse such emails, searching for terms like "secret" or "top-secret" or any mention of places with classified operations, such as Pakistan. Agents should also be able to figure out quickly how many of the emails duplicate those that have already turned up. "You could automate that pretty quickly," said one law enforcement official.

What will take more time, however, is making conclusions about whether any of the emails include classified information. That process, former FBI officials have said, could be cumbersome and drag on after the election. Investigators would have to read those for potentially relevant information, and, if there were questions about their classification, send them to other agencies for review.

But no one involved in the investigation is trying to delay, officials say.

"This is not a team that sits on its hands," said one official.

Abedin has told people that she is unsure how her emails could have ended up on a device she viewed as belonging to her husband, according to a person familiar with the investigation and civil litigation over the matter.

An announcement from the FBI in early October, when the emails were discovered, might have been less politically damaging for Clinton than one coming less than two weeks before the November 8 (November 9 NZT) election. The FBI declined to comment.

Comey wrote in his letter to Congress, "We don't know the significance of this newly discovered collection of emails," and federal law enforcement officials have said that investigators on the Clinton email team still had yet to thoroughly review them.

Comey in July announced that he was recommending that the investigation into Clinton's use of a private email server while she was Secretary of State be closed without charges. But he said investigators had found classified information on the server and characterised Clinton's and her aides' conduct as "extremely careless".

Legislators on both sides of the political aisle are likely to raise questions about why the team investigating Clinton's private email took so long to brief Comey. Clinton and her backers have pushed aggressively for the bureau to release more information about its findings and criticised the agency for making its work public without knowing more. Republican presidential nominee Donald Trump has called the matter "the biggest scandal since Watergate" and suggested, without evidence to support his claim, that the case against Clinton was now "so overwhelming".

A Washington Post-ABC News tracking poll found that more than 6 in 10 likely voters said the FBI's announcement would make no difference in their vote. A little more than 3 in 10 said the news made them less likely to support Clinton, though about two-thirds of those were Republicans or Republican-leaning independents.

Meanwhile, Senate Democratic leader Harry Reid sent a letter to Comey yesterday suggesting he violated the Hatch Act, which bars the use of a federal government position to influence an election.

"Through your partisan actions, you may have broken the law," Reid, a senator from Nevada, said in the letter to Comey.

And Richard Painter, a professor at the University of Minnesota Law School and the chief White House ethics lawyer from 2005 to 2007, said he filed a complaint over Comey's actions with the Office of Special Counsel, which investigates Hatch Act violations.

"We cannot allow FBI or Justice Department officials to unnecessarily publicise pending investigations concerning candidates of either party while an election is under way. That is an abuse of power," he said in a column in the New York Times.

However, Daniel Richman, a former federal prosecutor who now teaches at Columbia Law School, called the allegations that Comey improperly tried to influence the election "inane".

"Comey's critics cannot show his letter violated the Hatch Act unless they can prove that the FBI director was intending to influence the election rather than inform Congress, which was Comey's stated aim," said Richman, who said he had advised Comey on law enforcement policy but not this issue.