The FBI will have to sort through thousands of newly discovered emails in its renewed examination of the practices of Hillary Clinton and her aides, a U.S. official said Monday, raising questions about whether any findings might be released before election day.

The Justice Department, moving to address concerns over the timing of the revelation of the emails and a potential post-election spillover, said Monday it would "dedicate all necessary resources" to concluding the review promptly.

The timing matters because Donald Trump has been assailing the existence of the emails revealed in a remarkable and ambiguous letter to Congress from FBI Director James Comey last Friday. He said agents would take steps to review the messages, which were found on a computer seized during an unrelated investigation involving the estranged husband of a Clinton aide.

Anthony Weiner, the disgraced former New York congressman, is being investigated in connection with online communications with a teenage girl. He separated this year from Huma Abedin, one of Clinton's closest advisers.

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Republican presidential nominee Donald Trump, seen here at an airport campaign rally in Albuquerque, N.M., on Sunday, sees the FBI's actions as an opportunity to gain much-needed traction for his campaign. (Carlo Allegri/Reuters)

At the White House, spokesman Josh Earnest said he would neither defend nor criticize the timing of Comey's disclosure. But he also said President Barack Obama does not believe Comey was trying to influence the election, or strategizing to benefit one candidate or party.

"He's in a tough spot, and he's the one who will be in a position to defend his actions in the face of significant criticism from a variety of legal experts, including individuals who served in senior Department of Justice positions in administrations that were led by presidents in both parties," Earnest said.

It was not immediately clear exactly how many emails have been recovered or what significance, if any, they might have. But the U.S. official who spoke to The Associated Press said the trove numbers in the thousands and the FBI, which had a warrant to begin the review, would be focusing on those deemed pertinent to its earlier Clinton email server investigation.

In its letter to lawmakers, the department promised to "continue to work closely with the FBI and together dedicate all necessary resources and appropriate steps as expeditiously as possible."

The FBI and Justice Department closed that investigation, which looked into whether Clinton and her aides had mishandled classified information, without charges in July.

The official who spoke to the AP was not authorized to discuss an ongoing investigation by name and spoke on condition of anonymity.

Clinton is challenging the new email inquiry, saying "There is no case here."

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Clinton maintains the FBI should look at the emails and is confident they will clear her of wrongdoing again. (CBC News )

Clinton says she's "not making excuses" for her use of a personal email address and personal internet server at the State Department. And she says if the FBI wants to investigate emails involving Abedin, "they should look at them."

But she says she's sure the FBI will reach the same conclusion it did earlier this year, when the bureau decided against prosecuting Clinton and her advisers for their handling of classified information.

Donald Trump continued to use the emails to attack his opponent, saying this investigation could last for years.  

"Her election would mire our government and our country in a constitutional crisis that we cannot afford," Trump declared in Grand Rapids, pointing to the FBI's renewed examination as evidence the former secretary of state might face a criminal trial as president.

How did Abedin not know?

However, the fact that another cache of emails potentially important to the investigation has only recently been discovered raises an immediate question: How could Abedin have been unaware of their existence?

The answer is not yet clear, but it's possible that either she did not know about the emails on the computer of her estranged husband, forgot about them or for some other reason did not turn them over.

In a sworn deposition taken in June as part of a lawsuit filed by the conservative legal group Judicial Watch, Abedin was asked about what devices she had used to send or receive messages from her account on the clintonemail.com server. As part of the process in 2015 of returning her work-related emails to the State Department, Abedin said she "looked for all the devices that may have any of my State Department" work and provided two laptops and a Blackberry to her lawyers for review.

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A U.S. official told The Associated Press that the newly discovered emails emerged through the FBI's separate sexting probe of former congressman Anthony Weiner, the estranged husband of close Clinton confidant Huma Abedin. (Richard Drew/Associated Press)

For more about the Anthony Weiner texting scandal, watch the documentary WEINER this weekend on The Passionate Eye.

It airs Saturday and Sunday at 10 p.m. ET on CBC News Network.

Abedin made no mention of there being additional devices where her emails might have been saved.

"I was not involved in the process," Abedin said. "I provided them with the devices and the materials and asked them to find whatever they thought was relevant and appropriate, whatever was their determination as to what was a federal record, and they did.

They turned materials in, and I know they did so. I couldn't tell you from what device."

Abedin went on to say that she also provided her lawyers with her login and password to access her account on the Clinton server, which she said she used for all work-related matters while serving at the State Department.

If the FBI finds emails Abedin sent or received through the clintonemail.com server archived on the device recently recovered from her home, that would appear to conflict with what she told the FBI earlier this year.

However, Abedin added that after she left the State Department in 2013, Clinton's staff transitioned to a different email server and she "lost most of her old emails as a result." Abedin said she had only accessed her clintonemail.com account through a web portal and that she "did not have a method for archiving her old emails prior to the transition."

A person familiar with the investigation said the device that appears to be at the centre of the new review belonged only to Weiner and was not a computer he shared with Abedin. As a result, it was not a device Abedin searched for work-related emails at the time of the initial investigation, according to the person, who said of Abedin that it was "news to her" that her emails would be on a computer belonging to her husband.