The race clock showed 4:09:43 - the precise division between order and chaos, joy and panic, life and death. The first blast, just off the finish line, was strong enough to stiffen flags along the final yards of the course. Thirteen seconds later came another.
Three people were killed and 264 hurt. Over the next five days, the country learned that pressure cookers make easy bombs, joined an enormous manhunt, and watched as an entire city was locked down, until the surviving suspect was cornered in a boat.
The bombing of the Boston Marathon endures as the biggest event in a dizzying year of news.
A new pontiff in town
Under the dome of the Capitol, lawmakers brought the federal government to a halt, and the dysfunction was all too familiar. Under the dome of St. Peter, the pope resigned, and it hadn't happened since the Middle Ages.
In an announcement that shocked Catholics the world over, Benedict XVI announced in February that he no longer had the mental or physical strength to do the job. Less than three weeks later, he lifted off in a helicopter for Castel Gandolfo and retirement.
His successor was Jorge Mario Bergoglio, a little-known cardinal from Argentina who struck an utterly different and populist tone - emphasizing pastoral attention, humility and a personal touch. The change delighted lapsed Catholics but rankled conservatives.
The new pope took the name Francis. He encouraged Catholics to shake off an "obsession" with gay marriage and abortion. And he urged a "prayer for peace" in inflamed parts of the world that badly needed the help.
Those included Egypt, which was wracked by violence after its first democratically elected president was ousted by the military, and Syria, where nerve gas was unleashed on the people and the toll from a civil war surpassed 100,000.
The disasters that horrified the world were not just manmade.
Nature announced itself with fury in the Philippines, where a typhoon, perhaps the most ferocious ever to strike land, killed more than 6,000 people and turned a whole airport into a ragged rubble triage field.
Nature barely announced itself at all in Moore, Okla., where a tornado materialized one Monday afternoon in May, tore a path more than a mile wide and all but destroyed a town.
Twenty-five people were killed, including seven third-graders who were crushed when the twister reduced Plaza Towers Elementary School to a husk. Teachers there tried to calm the kids by singing "Twinkle, Twinkle, Little Star."
Rhonda Crosswhite, a teacher at Plaza Towers, remembered lying on top of her students to protect them: "One of my little boys, he just kept saying, 'I love you, I love you, please don't die with me.'"
Her students all made it.
Wrestling with guns and surveillance
Great debates consumed the country in 2013. One was about guns, and the nation learned the names of the Washington Navy Yard and Arapahoe High School and Sparks Middle School.
In the year after Newtown, at least 173 children were killed by gunfire.
Over the summer, the country watched a Florida courtroom and talked about firearms and race and self-defense. In the trial of the year, George Zimmerman was acquitted in the shooting death of Trayvon Martin, an unarmed black teenager.
The second was about the reach of technology, the power of government and the balance between privacy and security. A former government contractor named Edward Snowden unmasked himself and the details of National Security Agency spying programs.
Piece by piece, Snowden revealed a sprawling government surveillance operation that collects data on every telephone call made in the United States and on the emails of Americans and foreigners alike.
The White House defended the spying as essential to stopping terrorist attacks. In an extraordinary moment, President Barack Obama tried to calm public anxiety by insisting that the program was limited and saying: "Nobody is listening to your telephone calls."
Snowden, meanwhile, became the subject of international intrigue. He fled first to Hong Kong and ultimately to Russia, which granted him temporary asylum, to the frustration of American officials.
"My greatest fear was that no one would listen to my warning," he wrote in December, and it was long since clear that plenty of people had, including a federal judge who said that the NSA was probably violating the Constitution.
The number of states where gay marriage is legal doubled this year, to 18 - a figure that would have seemed unlikely if not impossible even five years ago. Those in support included, for the first time, a majority of the United States Senate.
And in June, the Supreme Court struck down the Defense of Marriage Act, which had blocked federal recognition of gay marriage. Edith Windsor, the 84-year-old plaintiff in the case, was asked what her late wife would have said about the landmark. She surmised: "You did it, honey."
Other Supreme Court rulings had far-reaching effects: The justices ruled that human genes could not be patented, upheld the power of police to take DNA swabs of people under arrest, and imposed atough standard for affirmative action in college admissions.
And in perhaps its most controversial decision of the year, the court struck down a key provision of the Voting Rights Act, considered to be the most important piece of civil rights legislation ever passed by Congress.
"There is no denying that, due to the Voting Rights Act, our Nation has made great strides," Chief Justice John Roberts wrote. Not far enough, said four justices, the president and a chorus of civil rights activists.
A breakdown in Washington
Obama took the oath of office 21 days into the new year and began a second term. From the west front of the Capitol, he proclaimed: "For now decisions are upon us, and we cannot afford delay."
In the next line, he decried absolutism, spectacle and name-calling, but in Washington this year there was plenty of all three.
Next to nothing got done. The Senate passed immigration reform, but it stalled in the House. To shouts of "Shame on you!", the Senate failed to pass a bill, overwhelmingly supported by the public, to expand background checks for gun purchases.
Obama's health-care overhaul launched on Oct. 1, but its federal website, healthcare.gov, was plagued by technical problems. And the president himself came under intense criticism for having promised that Americans who liked their insurance plans could keep them.
He acknowledged that the administration had "fumbled the rollout" of the health law. Of his own pledge, he said: "There is no doubt that the way I put that forward unequivocally ended up not being accurate."
For 16 days in October, the government itself shut down, forcing hundreds of thousands of federal workers to go without paychecks and doing an estimated $24 billion in damage to the economy.
With a catastrophic national default looming, Republicans insisted on delaying or defunding Obama's signature health law. They came away from the fight with only small concessions.
A beleaguered House Speaker John Boehner offered: "We fought the good fight. We just didn't win." Obama, for his part, made a rare point of bipartisan agreement: "The American people are completely fed up with Washington."
'Through hell and back'
The country at times seemed in short supply of heroes this year, and once in a while they emerged from unusual places.
A manhunt in California for Christopher Dorner, a rogue ex-police officer who had killed four people, ended when Karen Reynolds, whom Dorner had tied up with her husband, broke free and called 911.
"She got her gag off and then we both worked on trying to stand up," said the husband, Jim Reynolds. "I thought he was going to kill us."
In Cleveland, Amanda Berry waited until Ariel Castro, her captor of a decade, was out of the house, then kicked through a door and called for help. It arrived in the form of Chuck Ramsey, a neighbor whose enthusiastic retelling won him Internet fame.
Castro was sentenced to life plus 1,000 years for imprisoning, raping and torturing three women. Weeks later, he was found hanging by a bed sheet in his prison cell. The women returned home to adulation in Cleveland.
"I may have been through hell and back," said one of them, Michelle Knight, "but I am strong enough to walk through hell with a smile on my face."
In other places, the heroes were plentiful, and gone too soon. Nineteen elite firefighters from a 20-man crew known as the Granite Mountain Hotshots were trapped and killed by a lightning-sparked, wind-whipped wildfire in Arizona in July, the worst loss for an American fire department since Sept. 11.
"I wouldn't have traded the years I spent with those men for anything in this world," the only survivor, Brendan McDonough, told the local newspaper. "They made me the man and father I am today."
If you looked close enough, there were glints of hope to be found even in the impossibly dark stories of 2013.
In the Philippines, while corpses dangled from the branches of trees, a baby was born in the doorway of a church where most other prayers seemed unanswered. Another child met the world, to cheers,amid shards of glass from an air-traffic control tower.
And James Costello, whose arms and legs were so badly burned at the Boston Marathon bombing that he needed skin grafts, started planning a wedding. He will marry Krista D'Agostino, a nurse he met in rehab.
They had mutual friends but had never themselves crossed paths. She changed the dressing on his leg one day. Conversations turned into dates, and then "a few inseparable months," he wrote on Facebook. She said yes in France, during a cruise for first responders and victims from the bombing.
"One thing that she hates that I always say is I'm actually glad I got blown up," Costello said as the year came to a close. "I wish everyone else didn't have to, but I don't think I would have ever met her if I didn't, so I'm pretty happy."