The marathon was not the original target, the law enforcement officials said. The brothers had set their sights on a suicide attack on the city's massive Independence Day celebration, which draws about 500,000 people and is televised nationally.
But the bombs were ready sooner than anticipated, and a day or two before the Boston Marathon, they changed their plans, the officials said.
They spoke on the condition that their names not be used because they were not authorized to discuss the matter with the media.
One key question involves whether intelligence, law enforcement and homeland security agencies could have done more in their investigation of Tamerlan Tsarnaev in recent years before the attack and whether they shared information effectively enough. An independent government review is under way.
In an interview with Univision published on its website on Saturday, President Barack Obama said that cooperation among agencies can always get better. But he again defended U.S. efforts.
"I think we can continue to improve and refine how we're engaging and countering terrorist activity," Obama said. "I don't think it's fair to say though that law enforcement dropped the ball."
It is very difficult to prevent attacks when dealing with individuals who are self-radicalized and not part of a massive conspiracy or network, he said about the current belief of investigators looking at the origins of the Boston bombing.
Tamerlan's widow; Dzhokhar's friends
Tamerlan Tsarnaev's widow, Russell, has remained largely out of view since her husband's death, staying in her parents' Rhode Island home.
Her attorney, Amato DeLuca, says the 24-year-old knew nothing about plans to bomb the race, and reports of her husband's involvement came as an "absolute shock" to Russell and her family.
Three of Dzhokhar Tsarnaev's friends have been accused of covering for the bombing suspects.
Azamat Tazhayakov and Dias Kadyrbayev, both from Kazakhstan, were charged Wednesday with conspiring to discard potentially incriminating items from Dzhokhar Tsarnaev's dorm room. Robel Phillipos, a U.S. citizen, was charged with making false statements to investigators.
All three are accused of removing items from Tsarnaev's dorm room after the bombings. According to FBI affidavits released this week, they left with the backpack and Vaseline -- which Tazhayakov believed could be used to make bombs -- and Tsarnaev's laptop.
Investigators found the backpack, fireworks and Vaseline in a landfill last week after a two-day search.
The laptop was turned over by Kadyrbayev on April 19, the same day the FBI raided the apartment he shared with Tazhayakov, Kadyrbayev's attorney Robert Stahl said.
The FBI is examining the laptop, two federal law enforcement officials told CNN.
Authorities have said they believe the Tsarnaev brothers acted alone but are investigating whether they could have learned from or been aided by terror groups, including groups overseas.