BOSTON — The anniversary of the Boston Marathon bombings started with a solemn wreath-laying ceremony Tuesday morning at the site of the twin explosions, the first tribute in a day dedicated to honoring the three people who died, the more than 260 people who were injured, and the first responders, doctors and nurses who helped them.

The ceremony was attended by the families of the three bombing victims — Martin Richard, Krystle Campbell and Lu Lingzi — as well as relatives of Massachusetts Institute of Technology police Officer Sean Collier, who was killed in the aftermath of the blasts.