Two bombs exploded today near the finish line of the Boston Marathon, killing two people and injuring more than 20 others.
The bombs went off well after the start of the men's race, so there were still several thousand runners who hadn't finished yet.
Reports say there was a loud explosion on the north side of Boylston Street, just before the photo bridge at the finish line. A few seconds later, another explosion about 100 metres away.
A senior U.S. intelligence official told the Associated Press that two other explosive devices were found nearby and dismantled.
It's not clear whether or not the explosions were related to terrorism, and as of this writing, no one had claimed responsibility.
Boston police are investigating, and U.S. Attorney General Eric Holder has ordered a full investigation by the Federal Justice Department.
A senior U.S. government official told The New York Times, that neither Boston police nor the F.B.I. had received any reports or threats related to the marathon and there was no warning today.
Here's a video pack put together by the Boston Globe.
According to the Boston Marathon website, nearly 27,000 people were listed as entrants in the race including 2,078 Canadians.
Prime Minister Stephen Harper said his thoughts and prayers were with the victims of the explosions. He said it's a sad day when such an inspiring event is clouded by "such senseless violence."
Michael Doyle, a web producer for Canadian Running magazine, was one of the Canadians in the marathon.
"I heard there were two explosions, one of them close to the finish line, another further up," he told CBC News. "It's [a] very surreal situation."
"As I was making my way closer to the scene here, it was a mixture of people who were very upset and crying, and who seemed to be cognizant of what was going on, and those who I think were completely unaware of the situation."
Another Canadian runner, Laura McLean of Toronto, told CBC News she heard two explosions outside the medical tent.
"There are people who are really, really bloody," McLean said. "They were pulling them into the medical tent."
Calgary's Kristen-Ellen Fleming had been watching the race near the finish and then went into a restaurant nearby.
"We were coming out of the restaurant and there were two big explosions and a bunch of smoke... It was just immediate chaos and mayhem and confusion," she said.
"Everyone was just in utter shock and looking around."
"Within a very short amount of time, minutes, there were just sirens everywhere. It just sounded like an absolute war zone. People were crying, on the pavement, like sitting down with their head between their knees. Everyone was on their phone, obviously calling loved ones to let them know they were okay."
Roupen Bastajian, a runner from Rhode Island who had just finished, told the Associated Press "I started running toward the blast. And there were people all over the floor."
"We started grabbing tourniquets and started tying legs. A lot of people amputated...," he said.
Cherie Falgoust, who was waiting for her husband who was in the race, told AP "I was expecting my husband any minute,"
"I don't know what this building is... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runners who were still on the course were re-routed into a family meeting area.
After the explosions, U.S. Federal Aviation Administration created a no-fly zone over the immediate area for flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.
And cellphone service was shut down in the Boston area to prevent any possibility of any remote detonations.
Police also told people to stay indoors and avoid crowds.
Anthony Roman, a security expert, told The New York Times that authorities in large cities are usually on the highest level of alert for events like a marathon.
He said police would typically weld manhole covers shut, and check out the entire route just before the race.
They would also have snipers on rooftops, helicopters overhead, and high powered cameras "focusing on the race with their advanced software network, reading license plates," he said.
Today's race started with 26 seconds of silence in memory of the victims of last year's school shooting in Newtown, Connecticut.
The Boston Marathon started in 1897 and is one of the six World Marathon Majors. About half a million people usually turn out to watch.
Canadians in Boston who need emergency assistance are being urged to contact the consulate general at (617) 247-5100.
Friends and relatives seeking information on Canadians in the area are asked to contact 1-800-387-3124 or firstname.lastname@example.org.
And Google has set up a "Person Finder" page for people who are trying to track down loved ones in Boston.
For the latest on this story through the evening, here's a few sites to check in with: