Secret Service agents walk with President Obama upon his arrival in Evansville, Ind., on Friday. (Kevin Lamarque/Reuters)

More than 60 percent of the threats against President Obama are made online, according to the Secret Service, posing a new set of challenges for an agency under fire for a series of critical security lapses.

Lawmakers and private security officials question whether the Secret Service has sufficiently adapted to a new social-media landscape in which it must sort through a blizzard of online references to the president, investigate those that raise flags and then reconcile them with the intelligence they are gathering on the ground.

“I don’t know if they’ve adapted to these new threats,” said Rep. Jason Chaffetz (R-Utah), who chairs the House Oversight and Government Reform subcommittee on national security. “The attacks are going to come, no matter what. Are there new and creative ways of detecting them? I’m not convinced they’ve tied those loops.”

Chaffetz noted that he was “pleasantly surprised” that in 2011 agents were immediately able to pick up a tweet a D.C. woman posted about a man shooting at the White House. But he questioned why that piece of evidence was not used to corroborate suspicions among several officers that shots had been fired. Instead, the agency forwarded the report to the U.S. Park Police for further investigation, and it would be four days before it was discovered that bullets had hit the White House.

“Why didn’t that show up in the system?” Chaffetz asked about the tweet.

The night bullets hit the White House — and the Secret Service didn’t know

During Obama’s first run for the presidency, the issue of clearest concern was his race, which made him a magnet for threats from people who thought it disqualified him from the office.

As Obama nears six years in the White House, the number of overtly racist threats have subsided but the threats in general continue. Today, the dominant theme of grievances against the president is government overreach, according to current and former Secret Service officials, as critics suggest Obama is abusing his power and trampling the Constitution.

Brian Leary, a spokesman for the Secret Service, said the agency has adjusted to the fact that most threats against the president now occur online.

“The capability is there, and we have to evolve with technology as well,” he said, adding that the number of threats against Obama “did spike a few months after the [2008] election, but they declined back to a level that is consistent with his predecessors, and they still are.”

Other sources, who spoke on the condition of anonymity because of the sensitivity of the matter, said Obama received triple the number of threats than previous presidents during his initial candidacy and first year in office. That number has declined significantly since then, they said, but is still elevated compared with Obama’s predecessors

What constitutes a threat?

Members of the protective intelligence division consider even the most minor suggestions of harm to the president worthy of investigation. One agent, who spoke on the condition of anonymity to discuss internal agency operations, described being instructed to interview people who were intoxicated in a bar and were overheard describing how they would like to hurt the president.

Since Obama took office, at least 65 people have been indicted on charges of threatening to harm him. In January, Daniel L. Temple, who had tweeted “im coming to kill you” and “so I gotta kill barack obama first,” was sentenced to 16 months in prison after pleading guilty.

Nicholas Savino was sentenced in March to a year in prison for posting this on the White House Web site in August 2013: “President Obama the Anti-Christ, As a result of breaking the constitution you will stand down or be shot dead.”

Police, who arrested Savino a few days after he posted the statement, found three guns and about 11,000 rounds of ammunition in his apartment and car.

Agents briefed on protective intelligence for presidents and presidential candidates say that the rise in threats has much to do with the advance of the technological age, with the agency now receiving a much larger number of electronic communications that contain threats.

Today, racially based threats constitute 5 to 10 percent of the those made against the president, said people familiar with the matter.

Marilyn Mayo, co-director of the Anti-Defamation League’s Center on Extremism, said her group has found that physical threats against Obama and racial remarks on white-supremacist sites peaked in 2008 and 2009.

During the early days of Obama’s initial candidacy and the first year of his presidency, according to several people familiar with the matter, many of the threats against him had a frightening racist quality.

“If you had seen the stuff we were reading, it would have made your jaw drop,” said one former agent who spoke on the condition of anonymity because of the sensitive nature of the topic.

Mayo said her group continues to see “a tremendous amount of anger against Obama,” adding that much of it focuses on assertions that he has overstepped his constitutional authority.

Some critics do turn violent. Jerad Miller had called for Obama’s impeachment on his Facebook page; in June he and his wife, Amanda, shot two police officers in a pizza restaurant in Las Vegas. They placed a swastika and a “Don’t tread on me” flag on one officer’s body and a note on the other’s that read, “This is the start of the revolution.” Miller died in a shootout with police, while his wife committed suicide.

Mark Potok, a senior fellow at the Southern Poverty Law Center, which monitors white-supremacist organizations, said in an interview, “The fact that all of this is online makes the job in some way easier and in some way harder.

“All this ugliness is exposed to the light of day,” Potok said. “On the other hand, you typically have no idea who are the people posting on these sites, because they’re anonymous.”

Steve Atkiss, who served as special assistant for operations under President George W. Bush and is now a partner at Command Consulting, said his firm’s private and government clients are looking for ways to mine social media for these kinds of threats.

“One thing those clients have been clamoring for, for several years now, is a tool that would sort through the 50,000 tweets per second that are flying through cyberspace to find what is meaningful to them,” said Atkiss, adding that his firm has recently found software that puts keywords in context. “It’s been a struggle.”

One of the biggest challenges is finding the context to any violent or negative reference to the president, especially if agents are trying to track a plot in progress. In June, the Secret Service issued a work order seeking software that could detect sarcasm and identify social-media influencers online.

Early fears for Obama

Before Obama was elected president, the threats his Senate office received were so vituperative that Sen. Richard J. Durbin (D-Ill.) and Majority Leader Harry M. Reid (D-Nev.) asked the Secret Service to review whether he needed federal protection. Then-Secret Service Director Mark Sullivan told Congress that the agency was going to have to “pick him up” earlier than any presidential candidate in history, with the exception of Hillary Rodham Clinton, who had an existing detail because of her husband’s time in the Oval Office.

The prospect of the first African American president being assassinated was such a serious concern among some black voters that the Obamas addressed it directly in the campaign. Rep. James E. Clyburn (D-S.C.) said he discussed the matter with Michelle Obama after a high school teacher in his home town of Sumter told him that some of her African American students “were not going to support him. They did not want to see him elected, because somebody would kill him.”

Michelle Obama — who privately talked to Coretta Scott King, the widow of the Rev. Martin Luther King Jr., about the possibility of a presidential assassination before her husband was elected — gave an answer similar to one she gave in a 2007 interview with CBS’s Steve Kroft: that her husband was just as vulnerable as a private citizen.

“I don’t lose sleep over it, because the realities are, as a black man, Barack can get shot going to the gas station, you know,” she said in the interview. “So you can’t make decisions based on fear and the possibilities of what might happen. We just weren’t raised that way.”

Secret Service officials also ask aides to Cabinet members to notify the agency if they receive any menacing communications that have racial overtones and mention the president, according to people familiar with the matter.

Government overreach has been a major theme in this year’s midterm campaigns as Republicans have accused Obama of overstepping the bounds of his executive authority. It also has had a violent tone at times, such as Nevada rancher Cliven Bundy’s campaign to take up arms against Bureau of Land Management officials who sought to round up his cattle after he refused to pay federal grazing fees for years.

According to the Southern Poverty Law Center, the number of anti-government militia groups in the United States — which reached 858 during the Clinton administration — had dipped to a low point of 131 in 2007 under Bush. But it rose to 1,096 last year, a nearly tenfold increase since Obama took office.

Rep. John Lewis (D-Ga.) said in an interview that “there’s a meanness in American society today that reminds me of the period in our history, the civil rights period, where it was dangerous to speak up and speak out.”

But Lewis, who spoke with the Obamas at the Congressional Black Caucus Foundation’s recent annual dinner, said the revelations about potential vulnerabilities in the White House’s security system did not seem to be affecting the first couple.

“I don’t think they live in fear. None of us live in fear,” Lewis said, referring to prominent African American political figures. “You’re not preoccupied with what might or might not happen. You just do what you have to do.”

Carol D. Leonnig and Alice Crites contributed to this report.