One way Americans are showing their concern online is through a combination of images and memes.
The following is a list of the pictures we have seen.
Shared by @celtics on Instagram with the following message: "The Celtics family sends its thoughts and prayers to the friends and families affected by today's tragedy in Boston. Out of respect to all those who have been impacted by today's tragic events, the April 16 #Celtics game has been canceled."
This picture of eight-year-old Boston Marathon bombing victim Martin Richard was shared by Lucia Brawley on Facebook along with these thoughts: "This is Martin, 8. He died in the Boston bombing yesterday. He was at the finish line with his family, waiting for his dad to cross. His mother and little sister were catastrophically injured. He was the student of our dear friend, Rachel Moo. His message resonates powerfully today. My prayer is that we all live by Martin's words, paying tribute to his too-brief, but immeasurably valuable life by following his example."
Shared by @GeorgeTakei on Twitter based on a quote from Mr. Rogers with the message, "For Boston, and for hope."
Shared by @ball_err on Twitter with this comment: "Waking up you don't know if it;s gonna be your last. #prayforboston"
Shared by @OLBLightBrigade on Twitter with this expression of support: "Solidarity with the people of Boston sent from #lightbrigade Hilo, Hawaii. #BostonMarathon"
Shared by @kjpsi on Storify featuring an image of former Patriots guard Joe Andruzzi coming to the aid of a victim.
Shared by Rivky Jacobov on Instagram with this message: "I never would've imagined that the thing I love most in the world — running and specifically the marathon, an amazing event nevertheless, my marathon would turn into a tragic crime scene. But us marathoners are tough. We fight during training, we fight during the race. ... My heart goes out to everyone who was not as lucky as I was in avoiding the finish by minutes."
Shared by Deseret Book on Facebook with this suggestion: "Today may be a good day to send a message of hope to a friend."
The Chicago Tribune created a Boston tribute for the front page of today's sports section. Pictures of the cover are being shared on Facebook, Twitter and Instagram.