Information is still coming together, but the preliminary reports are horrible.
Two explosions went off near the finish line of the Boston Marathon on Monday, witnesses told NBC News.
The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners. Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the explosions.
Will Ritter, the spokesman for a Massachusetts Senate candidate, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.
"We heard two really large explosions in rapid succession, about a second apart from each other," Ritter said. "Everybody kind of ducked and hit the ground."
As I said, all of the news is preliminary, but there are some images of the apparent explosions available through social media, including this one. One local report said the blasts happened at about 2:50 p.m., leading to "dozens of injuries" and blowing out store fronts. There are also reports of "several lost limbs."
There's also this live feed of a CBS camera's live view of the finish line. Note that the area was evacuated after the blasts.
Update: According to the Boston Police Department, there were two fatalities and 23 injuries.
Second Update: NBC News is reporting that a small improvised explosive device is behind the finish-line blast at the Boston marathon -- the preliminary assessment of three separate law enforcement officials with an expertise in explosives. The report added, "It is still early in the investigation and forensic information is far from complete. A word of caution, while there are reports of multiple explosions, while that is not ruled out, there are often reports of multiple explosions that turn out to be reverberations from a single blast. Again, officials who spoke to NBC believe the explosion was at least one small improvised explosive device -- a homemade bomb."
Third Update: The street-level video of the explosion, from the Boston Globe, is obviously powerful. Note some of the people in fatigues rushing in around the two-minute mark.