Some acquaintances of the boys believe Dzhokhar Tsarnaev may have been brainwashed by his brother. "All I can say is I think his brother put him up to it," said Peter Tenzin, who co-captained the high school wrestling team with Dzhokhar. "There's no way in heck that he would do it. Mentally, he's just not that kind of guy."
4. Why wasn't the FBI tracking Tamerlan?
Acting on Russia's concerns that Tamerlan Tsarnaev was a follower of radical Islam, FBI agents interviewed the older brother in 2011, an FBI official said.
But the Russian government's request was vague and the lack of specifics limited how much the FBI was able to investigate Tsarnaev, according to a U.S. official and a law enforcement source.
Nevertheless, many are wondering why this young man seemingly dropped off the FBI's radar in the years since, particularly after his six-month trip to Russia's unstable North Caucasus region.
"This man was pointed out by a foreign government to be dangerous. He was interviewed by the FBI once ... then, he went to Russia. ... Why wasn't he interviewed when he came back either at the airport when he was returning or later?" Democratic Sen. Chuck Schumer of New York said to CNN.
Republican Sen. Lindsey Graham also voiced concern about the FBI's handling of Tamerlan Tsarnaev, but the senator from South Carolina pulled back his criticism on Monday, saying that the problem might have been partly the result of Tsarnaev's misspelled name on a travel document.
Many lawmakers are asking whether the FBI should have done more.
"I think there's going to be an awful lot of questions about this, that's going to be looked at over the coming months and maybe longer than that," said Rep. Michael Capuano, a Massachusetts Democrat.
"Like every situation, I have faith that the FBI and other agencies have done their job. Can they improve their job going forward? Absolutely."
5. Why was an MIT police officer killed?
This is another missing piece of the puzzle. Police don't know why Sean Collier, a young MIT campus police officer, was killed -- allegedly by the Tsarnaev brothers.
Collier was inside his police vehicle Thursday night after responding to a loud disturbance call when he was shot and killed on the MIT campus, according to Watertown Police Chief Edward Duveau.
"It appears that the suspects approached from the rear... and then fired four to five shots into the cruiser," Cambridge Police Commissioner Robert Haas said on "CNN Newsroom" Monday.
A short time after the MIT shooting, the suspects are believed to have carjacked an SUV about a mile away. Later, Tamerlan Tsarnaev was killed in a police confrontation.
The motive for Collier's killing is unknown, Haas said. "We really have no idea at this point in time."