After nearly a dozen years of foiled plots, the United States on Monday suffered the first large-scale bombing since the attacks of Sept. 11, 2001, opened an era of heightened security affecting nearly every aspect of American life.

The disruption of those plots underscores the enormous strides that the American national security apparatus has taken, including the adoption of policies that remain the subject of intense concern among human rights and civil liberties groups.

(Click for latest updates on the Boston Marathon bombings.)

But the success of the strike on the Boston Marathon, an international symbol of a city’s pride, highlights the enduring difficulty that U.S. officials face in impeding a determined attacker.

In remarks Monday evening, President Obama did not label the bombings as terrorism. But a White House official, who spoke on the condition of anonymity, said the incident was an “act of terror,” the same term that the president used in the aftermath of the attack on the U.S. diplomatic post in Benghazi, Libya, in September.

A former counterterrorism official said the Boston attack didn’t appear to have the signature of a coordinated al-Qaeda bombing, in which a sophisticated explosive device packed with shrapnel is detonated in an enclosed space to maximize casualties. The evidence could point to a domestic group, but White House officials and investigators cautioned that it was too soon to link the attack to any particular kind of perpetrator.

“At this stage, it’s perplexing,” said the former official, who would discuss an ongoing investigation only on the condition of anonymity. “It’s not a military or particularly iconic target like Times Square or the New York subway. This could be someone with limited or no foreign connections.”

From the FBI to local police departments, law enforcement agencies have dramatically shifted their emphasis to counterterrorism over the past decade, gathering intelligence on both domestic and foreign extremist groups. The George W. Bush and Obama administrations have created an enormous global apparatus designed to track and target terrorists.

But officials have always warned that the United States cannot prevent every attempted strike on U.S. soil. In some recent plots, authorities have benefited as much from luck as investigative skill.

The last mass terrorist killing on U.S. soil was carried out by Maj. Nidal M. Hassan, an Army psychiatrist, who fatally shot 13 people and wounded 30 more at Fort Hood, Tex., in November 2009. Hassan had connections to al-Qaeda in the Arabian Peninsula and the American-born Islamic cleric Anwar al-Awlaki, who was later killed by a U.S. drone strike in Yemen.

But there has been a series of failed or foiled bomb plots since the Sept. 11 attacks.

Less than three months after airliners crashed into the World Trade Center and the Pentagon, Richard Reid tried to detonate a shoe filled with explosives on a flight from Miami to Paris.

Eight years later, Umar Farouk Abdulmutallab attempted to set off explosives in his underwear on a commercial flight near Detroit.

The bombs failed to detonate correctly in both cases.

In two other incidents, authorities were able to prevent bombings.

A September 2009 attempt to set off bombs in the New York City subway system by an al-Qaeda associate was thwarted. And in May of the following year, Times Square in New York was evacuated after the discovery of a car bomb left by Faisal Shahzad, a dual citizen of Pakistan and the United States.

Since Obama’s election, there has also been huge growth in the number of anti-government “Patriot” groups. The Southern Poverty Law Center reported last month that because of the prospect of federal gun-control legislation, “the threat of violence appears to be looming.”

The center, which monitors hate groups, said that the number of Patriot groups reached record levels in 2012 and had grown by 813 percent over the past four years.

The movement is marked by a loathing of what adherents believe is a tyrannical federal government and a fear that the United States will be absorbed by some kind of global government.

In a letter to Attorney General Eric H. Holder Jr. last month, the Southern Poverty Law Center warned that “as in the period before the Oklahoma City bombing, we are now seeing ominous threats from those who believe that the government is poised to take their guns.”

The April 19, 1995, bombing of the Alfred P. Murrah Federal Building in Oklahoma City killed 168 people and was the largest terrorist attack in the United States before the Sept. 11 attacks.

When Timothy Mc­Veigh carried out the Oklahoma City bombing, the Patriot movement had been galvanized by the 1993 Brady Bill, the 1994 ban on assault weapons and the deadly conclusion of the standoff at the Branch Davidian compound outside Waco, Tex., among other events. The Southern Poverty Law Center has described the recent growth in Patriot groups as a second wave.

Because of the size of the crowd and because it was at a sporting event, the Boston attack also evoked memories of the bombing at the 1996 Atlanta Olympics. Two people were killed and more than 100 injured in an attack carried out by Eric Rudolph, who investigators described as an antiabortion and anti-gay extremist.

A report by the Combating Terrorism Center at West Point this year said that “there has been a dramatic rise in the number of attacks and violent plots originating from individuals and groups who self-identify with the far-right of American politics.”

But the report by the center’s director of terrorism studies, Arie Perliger, argued that there has been limited systematic documentation and analysis of incidents of American domestic violence.

Regardless of who carried out the Boston attack, it evoked for many the wrenching scenes of Sept. 11, especially among those who witnessed the midafternoon explosions.

Charlotte Holler, 24, was working seven stories above the scene in Boston’s Prudential Tower overlooking Boylston Street.

She heard two thuds and felt the explosions rock the floor under her feet. For an instant, she thought that her building or something nearby was collapsing.

When Holler dashed to the window to look down on the smoke-streaked street, she saw frightened marathon runners changing course to run away from the nearby finish line.

Holler said the building intercom system advised workers to stay inside. She did for a few hours — fearful, but also feeling lucky because she could reach her parents and friends on her land line and work computer to inform them she was safe.

“Everyone is harkening back to 9/11,” she said. “The priorities are: Where is my family? Where are my friends?”

Alice Crites, Carol D. Leonnig and Julie Tate contributed to this report.

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