The violence has left many grappling without answers, which President Barack Obama echoed in a hastily arranged address to the nation on Friday night.
"Why did these young men who grew up and studied here as part of our communities and our country resort to such violence?" the president said. "How did they plan and carry out these attacks? And did they receive any help? The families of those killed so senselessly deserve answers."
By Saturday afternoon, security across Boston remained tight.
Checkpoints were still visible in parts of the city, as were flower memorials set up near the marathon finish line, where three people suffered fatal wounds and more than 170 were injured.
But at Fenway Park, in an attempt to regain a sense of normalcy, more than 35,000 Red Sox fans bellowed out the lyrics to their eighth-inning signature tune, "Sweet Caroline," led personally in a surprise appearance by Neil Diamond.
"I bring love from the whole country," the 72-year-old singer told the crowd, drawing raucous cheers.
The Sox then rallied to beat the Kansas City Royals after trailing 2-1 in the top of the eighth.