This explainer is being regularly updated; click here for the latest post or jump to these recent updates:

On Monday, two blasts were reported near the finish line of the annual 26.2-mile Boston Marathon, resulting in at least two three dead and scores 132 176 injured, according to the Boston Police Department and news reports. The explosions—the first of which was on the north side of Boylston Street—occurred roughly three two hours after the winners crossed the finish line. “There are a lot of people down,” runner Frank Deruyter of North Carolina told the Associated Press shortly after the explosions. The cause of the blasts were not initially known.

Here’s video of the incident, via MSNBC:

Here are two photos from the scene (warning: graphic):

Here is the initial update from the Boston Marathon, via Facebook:

From the Boston Herald:

“I saw two explosions. The first one was beyond the finish line. I heard a loud bang and I saw smoke rising,” said Herald reporter Chris Cassidy, who was running in the marathon. “I kept running and I heard behind me a loud bang. It looked like it was in a trash can or something. That one was in front of Abe and Louie’s. There are people who have been hit with debris, people with bloody foreheads.”

In response to this news, New York City counterterrorism units were dispatched. “We’re stepping up security at hotels and other prominent locations in the city through deployment of the NYPD’s critical response vehicles (CRVs) until more about the explosion is learned,” New York City Police Department Deputy Commissioner Paul J. Browne said in a statement Monday afternoon. Washington, DC, and Los Angeles security were also put on high alert. The White House is in contact with state and local authorities in Boston and Massachusetts. “Our prayers are with those people in Boston who have suffered injuries. I don’t know how many there are,” Vice President Joe Biden said while on a conference call about gun legislation, when he was informed of the blasts.

More from the Massachusetts Emergency Management Agency:

If you are trying to reach friends or family and can’t get through via phone, try texing instead (less bandwidth) — MEMA (@MassEMA) April 15, 2013

RedCross Safe and Well is active. Individuals can register themselvs as safe or search for loved ones at redcross.org/safeandwell — MEMA (@MassEMA) April 15, 2013

If you are concerned for a friend that ran the #BostonMarathon today, you can see their last check-in here: raceday.baa.org/individual.html — NowThis News (@nowthisnews) April 15, 2013

An intelligence official told the AP on Monday that two additional explosive devices were found at the Boston Marathon, and were being dismantled, but those reports were later refuted by law enforcement and government officials.

Via the New York Times, here is a street map of where the explosions occurred:

Here is footage of the initial blast near the finish line, via Boston.com

Here it is from the perspective of a runner headed toward the finish line:

And here is video of the second blast, which occurred about 10 seconds after the first blast:

At 2:50 p.m. today, there were simultaneous explosions that occurred along the route of the Boston Marathon at the finish line. These explosions occurred 50 to 100 yards apart. Each scene resulted in multiple casualties. At this point in time all the victims shave been removed from the scene, we have sent officers to hospitals to be in touch with family members and possible witnesses. We immediately activated a system of response that the Commonwealth of Massachusetts and federal government has in place for these types of incidents…We have at this point in time determined that there has been a third incident that occurred. An explosion that occurred at the JFK library. This is very much an ongoing event at this time. We are not certain if these incidents are related, but we’re treating them as if they are.

There were no injuries at the JFK library that the police know of, per commissioner Davis.

If you are trying to locate someone, call: 617-635-4500. If you have any information about the explosions, call: 1-800-494-TIPS.

Update: The incident at the JFK library was later determined to be unrelated to the initial explosions.

This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.

At this time all Amtrak trains are operating as scheduled. We will provide an update if this changes. We are increasing security at stations & track right-of-ways. We ask passengers to […] report anything suspicious to 1-800-331-0008 or 911.

Obama: “Boston is a tough, resilient town and so are its people.” — The Boston Globe (@BostonGlobe) April 15, 2013

Here’s video of the press conference:

Click here for the full text of Obama’s statement; the speech lasted about three-and-a-half minutes. Here’s an excerpt:

We still do not know who did this or why. And people shouldn’t jump to conclusions before we have all the facts. But make no mistake — we will get to the bottom of this. And we will find out who did this; we’ll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice. Today is a holiday in Massachusetts — Patriots’ Day. It’s a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected from the earliest days of our nation. And it’s a day that draws the world to Boston’s streets in a spirit of friendly competition. Boston is a tough and resilient town. So are its people. I’m supremely confident that Bostonians will pull together, take care of each other, and move forward as one proud city. And as they do, the American people will be with them every single step of the way.

Boston is such elite race, 90% runners are sub-4-hrs. Sparing thousands from attack. In NYC bombs @ 4-hr mark woulda been more deadly. — Paul Kane (@pkcapitol) April 15, 2013

Now there’s a story about the runner on the ground, Bill Iffrig, who got up and finished. You can see a picture of Tlumacki taking the picture here. John Eligon, one of the writers of the lede New York Times piece, had also just run the marathon and somehow managed to file this story.

Can everyone pray for my Son Jeff jr who was at the finish line today in Boston. He is in surgery right now with injuries to his legs. I just can’t explain whats wrong with people today to do this to people. I’m really starting to lose faith in our country.

On Twitter, there’s been a lot of discussion about the ethics of running the picture without blurring the young man’s face, as the Atlantic did for over an hour on its site before altering the image. The Washington Post chose to crop the image so the victim’s legs are visible only above the knee.

One of the responders in the photograph—the man in the cowboy hat—has been identified as Carlos Arredondo, a Costa Rican immigrant whose Marine son died in action in Iraq in 2004. The day he learned of his son’s death, Arrendondo ?locked himself in a van with five gallons of gasoline and a propane torch and set the van on fire. He survived, became a peace activist, and was among the spectators who rushed toward the fumes after the explosion today. After tying a tourniquet onto the young man’s legs and wheeling him past the finish line to emergency help, Arredondo, seen badly shaken and trembling in this video, gripping a small American flag drenched in blood, talks to some bystanders on the street about the explosion:

THE PRESIDENT: Good morning, everybody. I’ve just been briefed by my national security team, including FBI Director Mueller, Attorney General Holder, Secretary Napolitano, and my Counterterrorism and Homeland Security Advisor Lisa Monaco, on the attacks in Boston. We continue to mobilize and deploy all appropriate law enforcement resources to protect our citizens, and to investigate and to respond to this attack. Obviously our first thoughts this morning are with the victims, their families, and the city of Boston. We know that two explosions gravely wounded dozens of Americans, and took the lives of others, including a 8-year-old boy. This was a heinous and cowardly act. And given what we now know about what took place, the FBI is investigating it as an act of terrorism. Any time bombs are used to target innocent civilians it is an act of terror. What we don’t yet know, however, is who carried out this attack, or why; whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual. That’s what we don’t yet know. And clearly, we’re at the beginning of our investigation. It will take time to follow every lead and determine what happened. But we will find out. We will find whoever harmed our citizens and we will bring them to justice. We also know this — the American people refuse to be terrorized. Because what the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness, and generosity and love: Exhausted runners who kept running to the nearest hospital to give blood, and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets. The first responders who ran into the chaos to save lives. The men and women who are still treating the wounded at some of the best hospitals in the world, and the medical students who hurried to help, saying “When we heard, we all came in.” The priests who opened their churches and ministered to the hurt and the fearful. And the good people of Boston who opened their homes to the victims of this attack and those shaken by it. So if you want to know who we are, what America is, how we respond to evil — that’s it. Selflessly. Compassionately. Unafraid. In the coming days, we will pursue every effort to get to the bottom of what happened. And we will continue to remain vigilant. I’ve directed my administration to take appropriate security measures to protect the American people. And this is a good time for all of us to remember that we all have a part to play in alerting authorities — if you see something suspicious, speak up. I have extraordinary confidence in the men and women of the FBI, the Boston Police Department, and the other agencies that responded so heroically and effectively in the aftermath of yesterday’s events. I’m very grateful for the leadership of Governor Patrick and Mayor Menino. And I know that even as we protect our people and aggressively pursue this investigation, the people of Boston will continue to respond in the same proud and heroic way that they have thus far — and their fellow Americans will be right there with them. Thank you very much. And you can expect further briefings from our law enforcement officials as the day goes on. When we have more details, they will be disclosed. What I’ve indicated to you is what we know now. We know it was bombs that were set off. We know that obviously they did some severe damage. We do not know who did them. We do not know whether this was an act of an organization or an individual or individuals. We don’t have a sense of motive yet. So everything else at this point is speculation. But as we receive more information, as the FBI has more information, as our out counterterrorism teams have more information, we will make sure to keep you and the American people posted. Thank you very much, everybody.

Along those veins, the Washington Post‘s Greg Miller reports that early stories from the New York Post and others about a Saudi citizen being detained or treated as a suspect in the wake of the bombings were incorrect:

U.S. law enforcement officials said Tuesday that a Saudi national injured in the Boston Marathon bombing is regarded as a witness, not a suspect. The Saudi, who is recuperating at a Boston hospital, is in his 20s and is in the United States on a Saudi scholarship to study at a university in the Boston area.

While detectives and federal agents have started the laborious process of interviewing thousands of people in Boston, much of the work that is likely to be key to solving the Boston Bombing is technical and forensic. First, the FBI will stitch together hundreds of hours of video camera recordings from private and public surveillance and traffic cameras, as well as recordings made by private citizens attending the race. They will look for when the bombs might have been left behind and then examine the faces of everyone who was in the area around that time. They will try to put names to those faces, using facial recognition matching software, drawing on drivers license, passport, and visa databases. In the case of the Mossad operation in Dubai, the police in the United Arab Emirates were able to recreate most of the the assassination operation by using snippets from dozens of surveillance cameras. For the FBI in Boston, a similar process has now begun. Second, FBI and NSA will look at phone records…

Read the rest here.

When the explosion went off, Fiola and his group immediately went into tactical mode. “I did a count and told the younger soldiers to stay put,” Fiola says. “Myself and two other soldiers, my top two guys in my normal unit, crossed the street about 100 yards to the metal scaffoldings holding up the row of flags. We just absolutely annihilated the fence and pulled it back so we could see the victims underneath. The doctors and nurses from the medical tent were on the scene in under a minute. We were pulling burning debris off of people so that the medical personnel could get to them and begin triage.”

Read the whole story here.

The father of a man who was photographed being pushed away from the Boston Marathon bombing in a wheelchair says his son has had both legs amputated. Jeff Bauman says his son, 27-year-old Jeff Bauman Jr., is the man in an Associated Press photo taken shortly after the bombing. His father says on his Facebook page that his son had to have both lower limbs removed at Boston Medical Center because of extensive vascular and bone damage. He says his son also had to have another surgery because of fluid in his abdomen.

One of the men who rescued Bauman was identified as Carlos Arredondo—”the man in the cowboy hat.” Read our story on Arredondo’s heroism at the Boston Marathon here.

UPDATE 21, Tuesday, April 16, 5:43 p.m. EDT: Boston University has released a statement confirming that the third victim of Monday’s bombing was a BU graduate student who attended the race with two friends. The school is not releasing the student’s name for now, citing the family’s wish for privacy.

UPDATE 22, Wednesday, April 17, 10:35 a.m. EDT: A report from a Chinese state-run newspaper states that the third victim is Lu Lingzi, a Chinese graduate student who was attending Boston University.

UPDATE 23, Wednesday, April 17, 10:42 a.m. EDT: Here’s Amy Davidson of The New Yorker on the Saudi citizen wrongly labeled a “suspect.” An excerpt:

A twenty-year-old man who had been watching the Boston Marathon had his body torn into by the force of a bomb. He wasn’t alone; a hundred and seventy-six people were injured and three were killed. But he was the only one who, while in the hospital being treated for his wounds, had his apartment searched in “a startling show of force,” as his fellow-tenants described it to the Boston Herald, with a “phalanx” of officers and agents and two K9 units. He was the one whose belongings were carried out in paper bags as his neighbors watched; whose roommate, also a student, was questioned for five hours (“I was scared”) before coming out to say that he didn’t think his friend was someone who’d plant a bomb—that he was a nice guy who liked sports. “Let me go to school, dude,” the roommate said later in the day, covering his face with his hands and almost crying, as a Fox News producer followed him and asked him, again and again, if he was sure he hadn’t been living with a killer.

UPDATE 24, Wednesday, April 17, 10:59 a.m. EDT: Here are two photos of Martin Richard, the 8-year-old boy who was killed in Monday’s attack. A vigil was held for the family on Tuesday night in Dorchester, Mass.

UPDATE 25, Wednesday, April 17, 1:22 p.m. EDT: We’ll have more on this as details are confirmed:

UPDATED: Bombing suspect possibly identified. bostonglobe.com — The Boston Globe (@BostonGlobe) April 17, 2013

Contrary to widespread reporting, no arrest has been made in connection with the Boston Marathon attack. Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate. Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.

UPDATE 29, Thursday, April 18, 11:15 a.m. EDT: President Obama attending an inter-faith memorial for the victims of the bombing.

Watch live now: President Obama to speak at Cathedral of the Holy Cross in Boston nyti.ms/11h79fA — The New York Times (@nytimes) April 18, 2013

“I have no doubt you will run again. You will run again! Because that’s what the people of Boston are made of,” Obama said. “It should be pretty clear they picked the wrong city [to mess with]. Not here in Boston! Not here in Boston.”

Obama: “You welcomed me when I was a state senator and few people could pronounce my name right.” #boston — Asawin Suebsaeng (@swin24) April 18, 2013

update Click here to read the president’s prepared remarks. Here’s an excerpt:

Boston may be your hometown, but we claim it, too. It’s one of America’s iconic cities. It’s one of the world’s great cities. And one of the reasons the world knows Boston so well is that Boston opens its heart to the world. Over successive generations, you’ve welcomed again and again new arrivals to our shores — immigrants who constantly reinvigorated this city and this commonwealth and our nation. Every fall, you welcome students from all across America and all across the globe, and every spring you graduate them back into the world — a Boston diaspora that excels in every field of human endeavor. Year after year, you welcome the greatest talents in the arts and science, research — you welcome them to your concert halls and your hospitals and your laboratories to exchange ideas and insights that draw this world together. And every third Monday in April, you welcome people from all around the world to the Hub for friendship and fellowship and healthy competition — a gathering of men and women of every race and every religion, every shape and every size; a multitude represented by all those flags that flew over the finish line. So whether folks come here to Boston for just a day, or they stay here for years, they leave with a piece of this town tucked fimly into their hearts. So Boston is your hometown, but we claim it a little bit, too. (Applause.) I know this because there’s a piece of Boston in me. You welcomed me as a young law student across the river; welcomed Michelle, too. (Applause.) You welcomed me during a convention when I was still a state senator and very few people could pronounce my name right.

UPDATE 31, Thursday, April 18, 1:54 p.m. EDT: The New York Post is at the center of another Boston-related media controversy. The Guardian reports:

The New York Post on Thursday has printed on its cover an image of two men standing together at the marathon under the headline “BAG MEN: Feds seek these two pictured at Boston Marathon.” The image shows two young men, one with a duffle bag and one wearing a backpack, talking to one another. The problem – a very big problem, for any media organization that would aspire to meet the most basic standard of accuracy – is that neither man appears to be a suspect in this attack.

Yep.

UPDATE 32, Thursday, April 18, 2:36 p.m. EDT: The teen boy pictured on the controversial cover of the New York Post speaks out. ABC News has the story:

The teenage boy authorities once investigated as possibly being connected to the Boston Marathon bombing told ABC News today he was shocked to see his face pop up on television and all over social media. Salah Barhoun, 17, said he went to the police yesterday to clear his name after he found himself tagged in pictures online. He had just gone to watch the race, he said, but soon after the explosions, he was singled out by internet sleuths as looking suspicious. Federal authorities passed around images of Barhoun, attempting to learn more information about him, sources told ABC News… Barhoun’s younger brother, who declined to be identified, said that it made his mother “sick and upset” that her son had been connected to the tragedy.” “It made her think he had done something wrong,” the teen’s younger brother said. “My brother is not the bomber.”

Here’s the “BAG MEN” New York Post cover from this morning:

UPDATE 33, Thursday, April 18, 2:56 p.m. EDT: The editor of the New York Post said Thursday that the paper “stands by its story.” But it has taken remarkable heat, even by its usual standard, for the potentially damaging misinformation it has spread in the wake of the attack.

The NY Post put an innocent 17-year-old runner on its front page as a “potential bombing suspect.” Nice going. deadsp.in/3zIw5ME — Deadspin (@Deadspin) April 18, 2013

Is the New York Post trying to set a kind of “Worst Reporting In A Single Week” record or something? bit.ly/13kPOmf — Ta-Nehisi Coates (@tanehisi) April 18, 2013

Think of something that would be interesting if true. Now, blurt it out. Congratulations, you’re a @nypost editor! — pourmecoffee (@pourmecoffee) April 18, 2013

Perhaps the Post has started to get the message; it has since posted a follow-up story reporting that the “two men probed” in the Boston Marathon bombings have been cleared by investigators.

UPDATE 34, Thursday, April 18, 3:02 p.m. EDT: The Atlantic Wire has a solid rundown of the federal investigation. (Click if you need a refresher on the supposed and misreported “suspects.”) According to the Boston Globe, “[a]uthorities have clear video images of two separate suspects in the Boston Marathon bombings…and are planning to release the images today in an appeal for the public’s help in identifying the men.” The Globe cites a source reportedly briefed on the case. However, CBS News’ John Miller maintains that images will be released Thursday, but only of one “person of interest.” (This is a reminder to question everything you read or hear—particularly in early reports—about the Boston Marathon bombing.)

The bureau posted the photos online:

For continuing coverage of the FBI’s suspects, click here.