That post-9/11 quiet? It's over.

The blasts on Boylston Street were felt across the nation, shaking and sometimes shattering a fragile hope -- formed slowly in the years since 2001 -- that maybe it won't happen here. Not again.

Then it did: in the Athens of America; on a holiday that marks the beginning of U.S. independence; at the sporting event to which all Boston looks forward; on the day when winter in New England unofficially becomes spring.

And it happened in a way that many experts had predicted. Instead of another massive, intricate 9/11-style attack, a couple of well-placed bombs created enormous fear while hurting only as many people, as one analyst put it, as "a very bad bus accident."

The explosions hurled shrapnel and jagged glass across the street, felling runners and spectators at the venerable Boston Marathon. Participants and onlookers alike lay dazed in the street and on the sidewalks, many bleeding profusely, some missing limbs. In a video that captured the moment of the blast, cheers turned to terrified screams as panic swept over the crowd.

"We've been a calm island in a sea of trouble," said Thomas Whalen, a political historian at Boston University. "We think of this as something that happens somewhere else. And now it's happened here."

And if it could happen there, it could happen anywhere. Events like the Boston Marathon, which had seemed so vulnerable to terrorism in the wake of 9/11, had been successfully policed and secured, along with Super Bowl after Super Bowl, World Series after World Series. Holidays, from Independence Day to Halloween to New Years Eve, passed without incident.

Aside from the pre-9/11 bombing at the 1996 Atlanta Olympics, you had to look to fiction -- notably the Super Bowl plot in Black Sunday -- for a such a nightmare.

"This reawakens a lot of our worst fears from 9/11," said David Barlow, director emeritus of the Boston University Center for Anxiety and Related Disorders.

Barlow, an expert on the psychological impact of terrorism around the world, should know. He was at the marathon finish line area Monday afternoon. He left a half hour before the bombs went off.

For years, Barlow said, "We didn't know when the other shoe was going to drop. Now it has." He was at home in nearby Boston Common. "We're in lockdown," he said.

Since Americans "all share this joint history of the 9/11 attacks, we're more affected by this (the Boston bombing) than we would otherwise," Barlow said. "We' re all imprinted by what happened 12 years ago."

The explosions landed with a jolt across the United States.

People hardened by the terrorist bombing almost 18 years ago to the day in Oklahoma City, and seared again by 9/11, were glued to their televisions, laptops, smart phones -- whatever could help them make sense of the senseless. Twitter and Facebook became bulletin boards for breaking news as well as places of comfort and collective grieving.

Stephen Flynn, a homeland security expert and author of America the Vulnerable: How Our Government is Failing to Protect Us from Terrorism, said Boston may be the first domestic example of what intelligence analysts have long warned Americans to expect -- small-scale attacks that are harder to anticipate and prevent. It was unclear whether the attack was foreign or domestic terrorism.

Flynn, who likened Monday's incident to a severe bus accident, said such attacks require relatively little time, money, personnel and planning. "Since you can't prevent every act like this, the ability to respond when they do happen is critical," he said. That reaction, he said, ranges from treatment of the wounded to speedy restoration of services such as mass transit.

A volunteer EMT, who declined to give his name but wouldn't say why, said he was walking downtown when the explosions occurred. He headed to a medical tent to volunteer his services and witnessed "chaos" inside. Police tried to remove him before a doctor demanded he stay and help. His own shirt bloodied as he recalled the scene, he said he only treated spectators. Among the many wounded, some were missing limbs, he said.

'YOU EXPECT IT'

Americans across the land watched the carnage from afar with a disbelief only partly mediated by the experience of 9/11.

Minutes after bombs went off, software engineer Marty Cano was at a gas station near Fort Hood in Killeen, Texas. A clerk told him, "America's been bombed again." Cano rushed across the street to the Veterans of Foreign Wars post, where his grandfather is a member, and told the bartender to turn off the jukebox and turn on the TV.

"Sad to say, but pretty much in this day and age you expect it, whether it be every two years or five years, or whatever," Cano said of such an attack. "I'm still taking it in -- not sure what to think."

In New York, Lee Ielpi, a retired firefighter who co-founded the September 11 Families' Association, said the obvious: "The first thing we flash back to is 12 years ago."

Colleen Tretter was attending a fundraiser in Somerset County, Pa., less than 10 miles from where United Flight 93 crashed after passengers rushed hijackers on 9/11. Tretter, a marathon runner herself, said the incongruity of someone using that event hit her hard.

"As a runner, you always know that early April race -- you always look forward to it," she said of the Boston Marathon, one of the world's oldest and greatest events of its kind. "I was astonished. … I never thought of that as a dangerous place, beyond running the 26.2 miles."

It happened in the 26th and final mile of the Marathon, a mile that had been dedicated to the 26 shooting victims at Sandy Hook Elementary School in Newtown, Conn. And it happened on Patriots' Day, the state holiday that marks the first shots fired by the colonial Minutemen at British soldiers in 1775 at the battles of Lexington and Concord.

The day started out well. The Red Sox beat Tampa Bay at Fenway Park in a game that started early so fans could drift over from Kenmore Square to Copley Square to catch the end of the marathon.

Ben Beach was going for a record 46 consecutive Boston Marathon runs. But he couldn't finish this race.

"My brother made it to the 21-mile mark, 5 miles from the end," wrote his brother Randall Beach, a reporter and columnist for the New Haven Register. "He was not allowed to go any farther. The race was over."

Out at the city limits, on the downside of Heartbreak Hill, the runners' last steep challenge, Boston College students occupied the median strip of Commonwealth Avenue to cheer on the weary runners.

'MORE THAN JUST A RACE'

For an event so big -- tens of thousands of runners, hundreds of thousands of spectators -- it always seemed so safe.

"It's a day with a festival atmosphere. It's not a day when anybody would ever think of anything like this happening," said Larry DiCara, a former member of the City Council.

He continued, rhapsodic despite the tragedy: "It's more than just a race. It's a day when kids come home for the long weekend, when families get together and go to the race. … It's not a day when you get much work done in Boston."

Steve Dowsett, 25, from Newbury Port, Mass., was staying at the Fairmont Copley Plaza and could hear the blasts from his hotel room.

"It's really sort of sickening. It's so random. It's such a positive event," Dowsett said.

Although two of the jetliners hijacked by terrorists on 9/11 departed from Boston's Logan International Airport, the city has been relatively free of violence and terrorism jitters. "That's what makes this all the more striking," DiCara said.

Whalen, the Boston University historian, said the explosions made "sense as terrorism -- if you want to do maximum psychological damage. This is a day when you bring the kids out, everyone has a great time. No one worries. This might kill that."

For the young people of greater Boston, Marathon Day is a rite of passage, when they pile onto the T mass transit system with their pals and head to Copley Square. They realize they've arrived someplace, because the square is defined by some of the great monuments of world architecture -- McKim, Mead and White's Boston Public Library and H.H. Richardson's Trinity Church.

Whalen thought about those high school kids who had made the trek Monday from places nearby -- Jamaica Plain, Chestnut Hill, Revere: "What a brutal coming of age."

Contributing: Susan Davis and David Leon Moore in Boston