WASHINGTON — FBI Director James Comey recommended Tuesday that no criminal charges be brought against Hillary Clinton, ending a yearlong investigation into how she handled classified State Department information on her private e-mail server while she was secretary of state.

The finding almost certainly means the Democratic Party will avoid its worst-case scenario: a mug shot of their presumptive presidential nominee.

But Comey gave Clinton’s many critics plenty of ammunition, saying Clinton and her colleagues were “extremely careless in their handling of very sensitive, highly classified information.’’

Comey’s scathing assessment of her e-mail practices reinforced existing questions about her trustworthiness and a perception that she plays by her own set of rules.

His rebuke will be used to raise doubts about Clinton’s claims that her competency and judgment make her the most qualified candidate to be president of the United States.

In an unusual, public explanation of the FBI investigation, Comey also shredded Clinton’s frequently deployed talking points about the private server, saying she should have known some of the e-mails she received and sent were classified even though they lacked markings. He also contradicted her previous assertions that she turned over all work-related e-mails from the private server to the State Department, saying that the FBI found thousands that were not submitted and that countless more were probably permanently deleted.

The job of deciding whether to pursue an indictment still rests with the Justice Department, but Comey’s move makes it extremely unlikely any action will be taken. Comey said no “reasonable prosecutor’’ would bring a case.

Comey delivered his announcement just hours before Clinton was set to board Air Force One with President Obama for a trip to North Carolina, where the two appeared together on the campaign trail for the first time.

White House press secretary Josh Earnest said Obama and Clinton didn’t discuss the investigation or the FBI’s announcement on the trip. Earnest sidestepped questions about whether the president agrees with the FBI’s conclusion that Clinton was “extremely careless’’ with classified material.

No one at the White House was alerted about the FBI recommendation or that Comey would make his announcement Tuesday, the spokesman said. The North Carolina event proceeded as if nothing had happened as Obama — with jacket off and shirtsleeves rolled up — declared that “Hillary’s got me fired up!’’ and extolled her “judgment,’’ “toughness,’’ and “brilliance.’’

Donald Trump, the presumptive Republican nominee, attacked both Clinton and the FBI in a series of Tweets, seeking to frame the FBI’s recommendation not to pursue criminal charges as a symptom of the insider Washington culture that his campaign is trying to upend.

“FBI director said Crooked Hillary compromised our national security. No charges. Wow! #RiggedSystem,’’ he posted on Twitter.

House Speaker Paul Ryan, who is not always on the same page as Trump, said in a statement that the FBI’s decision “defies explanation’’ and that the findings “make clear that Secretary Clinton misled the American people when she was confronted with her criminal actions.’’

While damaging, Comey’s announcement likely will prove to be an inflection point in the scandals that have dogged the New York Democrat as she unrolled her presidential bid. Last week a Republican-led House Committee released a report that contained no new bombshells to implicate Clinton in wrongdoing or blame in the 2012 attacks in Benghazi, Libya, that killed four Americans.

Comey described his 15-minute statement as “unusual’’ for the FBI, which rarely even acknowledges investigations that don’t end with legal action.

“I think the American people deserve those details in a case of intense public interest,’’ he said.

Comey served as a high-ranking official in the Justice Department during the administration of George W. Bush and was nominated for the position of FBI director by Obama in 2013. He is widely respected on both sides of the aisle.

He painted a portrait of sloppiness and poor attention to security in Clinton’s handling of her e-mails. He said Gmail, the widely used and free e-mail service provided by Google, features better security and archiving functions than Clinton’s homemade setup, which was housed in her home in Chappaqua, N.Y.

He went on to say the “security culture at the State Department’’ was “generally lacking in the kind of care for classified information found elsewhere in the government.’’

It’s possible that “hostile actors’’ hacked into Clinton’s private e-mail account due to her practice of “sending and receiving work-related e-mails in the territory of sophisticated adversaries,’’ he added. Under current practices at the State Department, top officials use temporary cellphones and computers when traveling to places like China, where government agents are known to try to spy on diplomats.

Comey revealed that federal investigators found 110 e-mails on Clinton’ servers that included classified information. Eight e-mail chains included information that was “top secret’’ at the time they were sent; 36 chains that included “secret’’ information; and eight contained e-mails that were “confidential,’’ he said. Comey also revealed for the first time that Clinton used multiple private e-mail servers, apparently in sequence when previous units were replaced.

Even as Comey said that Clinton should not be prosecuted, he acknowledged that others have gotten in trouble for similar offenses.

“Although there is evidence of potential violations of the statutes regarding the handling of classified information, our judgment is that no reasonable prosecutor would bring such a case,’’ Comey said. He said the threshold of intentionally mishandling classified information or gross negligence had not been met.

“To be clear, this is not to suggest that in similar circumstances, a person who engaged in this activity would face no consequences,’’ Comey added. “To the contrary, those individuals are often subject to security or administrative sanctions. But that is not what we are deciding now.’’

Clinton has been dogged by questions about her e-mail set up since before she launched her campaign. She’s offered an array of reasons for using a private server and e-mail address instead of a government account, including that it was more convenient for her to carry a single device instead of multiple smart phones.

She initially claimed that no classified material was sent or received on her private server, but soon shifted to a more legalistic explanation she didn’t send or receive information “marked’’ classified.

Under court order, the State Department released thousands of Clinton’s e-mails over the past year.

Her representatives regularly argued that the classified ­e-mails redacted from that trove were made secret after the fact. They even complained about a “culture of classification’’ in the federal government that was excessive.

Brian Fallon, a spokesman for Clinton’s campaign, said in a statement that the campaign is “pleased’’ by the FBI’s recommendation not to bring ­charges.

“As the secretary has long said, it was a mistake to use her personal e-mail and she would not do it again,’’ Fallon said in a statement. “We are glad that this matter is now resolved.’’

Republicans wasted no time seizing on the FBI director’s criticism of Clinton’s e-mail practices.

America Rising, a conservative super PAC, quickly posted a Web video featuring clips of Clinton vowing that she didn’t send any classified material via her private e-mail followed by Comey’s statement that more than 100 with classified information was sent or received by the former secretary of state.

Republican National Committee chief strategist Sean Spicer called the FBI’s decision “absolutely ridiculous’’ during an interview with CNN shortly after the FBI press conference ended.

“Any other employee that works at the State Department that did what she did would have their security clearance pulled and would be facing charges,’’ Spicer said.

The Department of Justice, headed by Attorney General Loretta Lynch, can still determine whether to act. But Lynch has already said she will accept the FBI’s recommendation.

That process became fraught last week after a report surfaced that former president Bill Clinton met privately with Lynch on June 27 when their planes were parked at an airport in Phoenix. Clinton boarded Lynch’s plane and the two talked for about half an hour.

Both said that the investigation was not mentioned. But Lynch acknowledged that the meeting was “casting a shadow’’ over the process.

Annie Linskey can be reached at annie.linskey@globe.com. Follow her on Twitter @annielinskey.