Events are unfolding on the ground and online.
This story has been updated.
Here’s a quick inventory of the role social media is playing in the response to Monday's bomb explosions at the Boston Marathon finish line that has caused at least two deaths so far.
The Boston Police Department began tweeting confirmed news about 45 minutes after the first explosion @Boston_Police. The police department was one of thousands tweeting about the events under #bostonmarathon and related hash tags.
Web watchers also can follow the Boston police and emergency scanner online.
Racers themselves can be tracked at the Boston Marathon site through chips in their socks.
The Boston Globe has set up a Google document for stranded runners seeking housing and people willing to put them up.