Here’s what we know as of now:

There were two different explosions that took place about 15 seconds apart at the finish line at the Boston Marathon. The explosions were centered at two different locations so it is very likely they were no accident.

Somebody’s leg flew by my head,” a spectator, who gave his name as John Ross, told the Boston Herald. “I gave my belt to stop the blood.”
Witnesses heard booms that sounded like two claps of thunder near the finish line inside the Fairmount Copley Plaza Hotel, according to multiple local reports.

Video of the scene showed a number of emergency crews in the area tending to victims and blood on the ground near the finish line.

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

The explosions took place exactly 4 hours after the secondary runners started the race, so if it was a terrorist attack it was targeted to the regular joe-type runners.

There have been at least 3 confirmed dead and according to Fox News at least ten people are at the hospital needing a limb amputated.

Police are not allowing people to use cell phones for fear they will be used as ignition devices.

A third bomb was found down the street from the other two near the library. There were reports of smaller explosions (possibly other bombs found by the authorities and exploded on a controlled basis)

Check back here for confirmed updates.

4:30 According to the Boston Fire Department the first explosion took place in a third floor conference room of the Westin Hotel. The Second one seems to have happened on ground level.

Intelligence officials told The Associated Press two unexploded devices were being dismantled, and reports of a third “controlled” explosion may have been an intentional detonation supervised by authorities.

Massachusetts General Hospital said it was treating 19 injured people; Tufts Medical Center reported that some injured were to be treated there as well.

 CNN says 29 people are being treated at area hospitals. Below is the first video of the explosions:

The NY Post has confirmed that a suspect is being guarded at an area hospital:

Authorities have confirmed to The Post that they have identified a suspect in the Boston Marathon bombing. The suspect — a Saudi national who suffered shrapnel wounds in today’s blast — is currently being guarded in a Boston hospital. A law enforcement source confirmed to The Post that 12 people were killed and nearly 50 were injured in today’s blast.

There has been a third explosion (this one at the Kennedy Library) But there is some dispute whether it is related to the other two. 6:20 update : Explosion at JFK Library appears to be fire related not a bombing”

Boston Globe is reporting more than 100 people are being treated for injuries, citing local hospitals. Fox News is reporting a “person of interest” in the hosptial

Authorities were guarding a person of interest at a local hospital, sources told The person, who sources said was 20 years old, had severe burns, but authorities had not determined whether the person was a victim or a perpetrator. Boston Police Commissioner Edward Davis said during a press conference that no suspect is in custody. 

 8:00 PM update Just got home…Penn Station lined with police and national guard..I would say 3x the usual amount. President called the event a tragedy said the perpetrator will be brought to justice..Wrong! It was a terrorist attack and the perpetrator should be hunted down as an enemy combatant. Fox News reports that the president believes it is a terrorist attack but he didn’t call it that in his speech. Makes one wonder if he will end up calling this an athletic injury the way he calls Fort Hood a workplace injury. Boston Globe Reports Two Dead…one of the dead was an 8 year old boy.. source says; 132 injured, 17 critical. Doctors are “pulling ball bearings out of people in the emergency room,” suggesting that the bombs were designed to propel shrapnel, a terrorism expert briefed on the investigation told CNN’s Deborah Feyerick. NBC News legal correspondent Pete Williams reports that authorities have not yet identified anyone who may be responsible for Monday’s attack on the Boston Marathon. However, Williams reported that authorities do have a “young person” who is in the United States on a student visa in custody. That person is being questioned in relation to the bombings.  

8:20 UPDATE: NBC News says this is a picture of the suspect being arrested. Boston Globe Reports  just before 8 p.m., a “person of interest” was being questioned at Brigham and Woman’s Hospital in connection with the bombing according to an official briefed on the investigation. The individual was hurt in the explosion, the official said.

Same suspect from a different angle:

Helpful Map of the bombings in relation to the Marathon Finish Line

9:00 Update

CBS News senior correspondent John Miller reports that a Saudi national is being questioned by authorities. He was seen “acting suspiciously” running from the explosion, and a civilian chased him down and tackled him. He was turned over to Boston police and is being questioned by the FBI. He is being cooperative and denies any involvement.

“This could mean a lot, or this could mean very little,” Miller said. “It’s too soon to call him a suspect.”

Miller reported earlier that authorities are also reviewing surveillance video that shows a man from behind carrying two backpacks near the site of the explosions. Authorities are not sure whether the subject in the video is linked to the blasts.

Police commissioner, “Let me stress there is no suspect at Brigham and Woman’s Hospital, there are people being questioned but there is no suspect.” 

He also said there were three dead.

The commissioner said that there would be no updates till 9:30 in the morning.