(CNN)Overlooked in the controversy over Hillary Clinton's use of a private email server, is the fact that suspected Russian hackers have bedeviled State Department's email system for much of the past year and continue to pose problems for technicians trying to eradicate the intrusion.
Federal law enforcement, intelligence and congressional officials briefed on the investigation say the hack of the State email system is the "worst ever" cyberattack intrusion against a federal agency. The attackers who breached State are also believed to be behind hacks on the White House's email system, and against several other federal agencies, the officials say.
Clinton: 'Would've been better' to use official email 02:31
The issue is relevant because one criticism of the Clinton private email use is that it was likely less secure than emailing within the State Department's system. But the hack shows that State's system has major security issues. The State system, investigators believe, was compromised in the past year, likely after Clinton left the State Department.
Last November, the State Department shut down its email system over a weekend to try to improve security and block the intruders.
At the time, the agency tried to send a re-assuring message that "activity of concern" by possible hackers only affected its unclassified email system.
But officials say that even a breach of the unclassified system poses major security risks, because sensitive information of value to foreign intelligence agencies is routinely shared in non-classified emails.
Russian hackers, likely working for the Russian government, are suspected in the State Department hack.
The FBI has been investigating the hacking activity.
In part because of the Russian attack on State, U.S. intelligence officials have increased their warnings about Russian hacking activity in the U.S.
James Clapper, director of National Intelligence, told a Senate hearing last month the "Russian cyber threat is more severe than we have previously assessed."
Clapper's comments were in part prompted by the intrusion at State and other federal agencies, according to one official briefed on the matter.
Former Secretary of State Hillary Clinton said Tuesday that her use of a private email server didn't include any classified information. She said the computer server hosting her emails was in a private building protected by the U.S. Secret Service.
Officials say that doesn't necessarily protect against hackers who don't need physical access to the server. In a statement released by Clinton's office after the press conference Tuesday, a spokesman said there was "no evidence" the server was ever hacked.
Photos: Hillary Clinton's life in the spotlight
Hillary Clinton accepts the Democratic Party's nomination for president at the Democratic National Convention in Philadelphia on July 28. The former first lady, U.S. senator and secretary of state was the first woman to lead the presidential ticket of a major political party.
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Photos: Hillary Clinton's life in the spotlight
Before marrying Bill Clinton, she was Hillary Rodham. Here she attends Wellesley College in Massachusetts. Her commencement speech at Wellesley's graduation ceremony in 1969 attracted national attention. After graduating, she attended Yale Law School.
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Rodham was a lawyer on the House Judiciary Committee, whose work led to impeachment charges against President Richard Nixon in 1974.
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In 1975, Rodham married Bill Clinton, whom she met at Yale Law School. He became the governor of Arkansas in 1978. In 1980, the couple had a daughter, Chelsea.
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Arkansas' first lady, now using the name Hillary Rodham Clinton, wears her inaugural ball gown in 1985.
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The Clintons celebrate Bill's inauguration in Little Rock, Arkansas, in 1991. He was governor from 1983 to 1992, when he was elected President.
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Bill Clinton comforts his wife on the set of "60 Minutes" after a stage light broke loose from the ceiling and knocked her down in January 1992.
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In June 1992, Clinton uses a sewing machine designed to eliminate back and wrist strain. She had just given a speech at a convention of the International Ladies' Garment Workers Union.
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During the 1992 presidential campaign, Clinton jokes with her husband's running mate, Al Gore, and Gore's wife, Tipper, aboard a campaign bus.
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Clinton accompanies her husband as he takes the oath of office in January 1993.
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The Clintons share a laugh on Capitol Hill in 1993.
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Clinton unveils the renovated Blue Room of the White House in 1995.
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Clinton waves to the media in January 1996 as she arrives for an appearance before a grand jury in Washington. The first lady was subpoenaed to testify as a witness in the investigation of the Whitewater land deal in Arkansas. The Clintons' business investment was investigated, but ultimately they were cleared of any wrongdoing.
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The Clintons hug as Bill is sworn in for a second term as President.
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The first lady holds up a Grammy Award, which she won for her audiobook "It Takes a Village" in 1997.
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The Clintons dance on a beach in the U.S. Virgin Islands in January 1998. Later that month, Bill Clinton was accused of having a sexual relationship with former White House intern Monica Lewinsky.
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Clinton looks on as her husband discusses the Monica Lewinsky scandal in the Roosevelt Room of the White House on January 26, 1998. Clinton declared, "I did not have sexual relations with that woman." In August of that year, Clinton testified before a grand jury and admitted to having "inappropriate intimate contact" with Lewinsky, but he said it did not constitute sexual relations because they had not had intercourse. He was impeached in December on charges of perjury and obstruction of justice.
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The first family walks with their dog, Buddy, as they leave the White House for a vacation in August 1998.
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President Clinton makes a statement at the White House in December 1998, thanking members of Congress who voted against his impeachment. The Senate trial ended with an acquittal in February 1999.
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Clinton announces in February 2000 that she will seek the U.S. Senate seat in New York. She was elected later that year.
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Clinton makes her first appearance on the Senate Environment and Natural Resources Committee.
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Sen. Clinton comforts Maren Sarkarat, a woman who lost her husband in the September 11 terrorist attacks, during a ground-zero memorial in October 2001.
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Clinton holds up her book "Living History" before a signing in Auburn Hills, Michigan, in 2003.
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Clinton and another presidential hopeful, U.S. Sen. Barack Obama, applaud at the start of a Democratic debate in 2007.
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Obama and Clinton talk on the plane on their way to a rally in Unity, New Hampshire, in June 2008. She had recently ended her presidential campaign and endorsed Obama.
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Obama is flanked by Clinton and Vice President-elect Joe Biden at a news conference in Chicago in December 2008. He had designated Clinton to be his secretary of state.
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Clinton, as secretary of state, greets Russian Prime Minister Vladimir Putin during a meeting just outside Moscow in March 2010.
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The Clintons pose on the day of Chelsea's wedding to Marc Mezvinsky in July 2010.
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In this photo provided by the White House, Obama, Clinton, Biden and other members of the national security team receive an update on the mission against Osama bin Laden in May 2011.
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Clinton checks her Blackberry inside a military plane after leaving Malta in October 2011. In 2015, The New York Times reported that Clinton exclusively used a personal email account during her time as secretary of state. The account, fed through its own server, raises security and preservation concerns. Clinton later said she used a private domain out of "convenience," but admits in retrospect "it would have been better" to use multiple emails.
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Clinton arrives for a group photo before a forum with the Gulf Cooperation Council in March 2012. The forum was held in Riyadh, Saudi Arabia.
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Obama and Clinton bow during the transfer-of-remains ceremony marking the return of four Americans, including U.S. Ambassador Christopher Stevens, who were killed in Benghazi, Libya, in September 2012.
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Clinton ducks after a woman threw a shoe at her while she was delivering remarks at a recycling trade conference in Las Vegas in 2014.
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Clinton, now running for President again, performs with Jimmy Fallon during a "Tonight Show" skit in September 2015.
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Clinton testifies about the Benghazi attack during a House committee meeting in October 2015. "I would imagine I have thought more about what happened than all of you put together," she said during the 11-hour hearing. "I have lost more sleep than all of you put together. I have been wracking my brain about what more could have been done or should have been done." Months earlier, Clinton had acknowledged a "systemic breakdown" as cited by an Accountability Review Board, and she said that her department was taking additional steps to increase security at U.S. diplomatic facilities.
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U.S. Sen. Bernie Sanders shares a lighthearted moment with Clinton during a Democratic presidential debate in October 2015. It came after Sanders gave his take on the Clinton email scandal. "The American people are sick and tired of hearing about the damn emails," Sanders said. "Enough of the emails. Let's talk about the real issues facing the United States of America."
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Clinton is reflected in a teleprompter during a campaign rally in Alexandria, Virginia, in October 2015.
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Clinton walks on her stage with her family after winning the New York primary in April.
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After Clinton became the Democratic Party's presumptive nominee, this photo was posted to her official Twitter account. "To every little girl who dreams big: Yes, you can be anything you want -- even president," Clinton said. "Tonight is for you."
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Obama hugs Clinton after he gave a speech at the Democratic National Convention in Philadelphia. The president said Clinton was ready to be commander in chief. "For four years, I had a front-row seat to her intelligence, her judgment and her discipline," he said, referring to her stint as his secretary of state.
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Clinton arrives at a 9/11 commemoration ceremony in New York on September 11. Clinton, who was diagnosed with pneumonia two days before, left early after feeling ill. A video appeared to show her stumble as Secret Service agents helped her into a van.
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Clinton addresses a campaign rally in Cleveland on November 6, two days before Election Day. She went on to lose Ohio -- and the election -- to her Republican opponent, Donald Trump.
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After conceding the presidency to Trump in a phone call earlier, Clinton addresses supporters and campaign workers in New York on Wednesday, November 9. Her defeat marked a stunning end to a campaign that appeared poised to make her the first woman elected US president.
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