Boston Marathon was 'target of opportunity,' bombs built in attacker's home, sources say

Authorities suspect the Tsarnaev brothers built the explosives used to attack the Boston Marathon in the older brother's home and chose the prestigious race as a "target of opportunity," sources tell Fox News.

The older brother, Tamerlan Tsarnaev, 26, and 19-year-old Dzhokhar Tsarnaev allegedly assembled at least four types of improvised explosive devices in the home Tamerlan shared with wife Katherine Russell, sources tell Fox News. Typically, in cases of homegrown terror, only one type of explosive is constructed.

Additionally, Dzhokhar Tsarnaev told investigators the building of the bombs came together more quickly than expected, according to sources. The brothers reportedly had planned to set off the explosives on July 4, but changed their plans when the bombs were finished early.

After the Boston bombings, the brothers still had a pressure-cooker explosive and four pipe bombs and were bound for New York City before they were stopped by authorities, New York officials say. Tamerlan Tsarnaev was killed after a shootout with police days after the Boston attack. Dzhokhar was captured alive but wounded and now is in federal custody facing a charge of use of a weapon of mass destruction that could get him the death penalty..

Sources say the planning for the April 15 attack showed "clear premeditation," including the decision to target the finish line of the Boston Marathon. Three people died in the attack, and more than 200 people were wounded.

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    Some speculated that the attack was specifically timed for Patriot's Day, a key holiday in the city of Boston, but sources suggest to Fox News that the race was targeted because of its timing shortly after the bombers' felt prepared to carry out their plan.

    Three of Dzhokhar's friends were charged Wednesday with hindering the investigation, including dumping a laptop and a backpack filled with hollowed-out fireworks that were found in his college dorm room.

    Documents based on interviews with the three fellow students reveal Dzhokhar Tsarnaev allegedly dropped sinister hints before the attack, telling his friends a month before that he had learned how to make a bomb. However, it wasn’t until the FBI released a surveillance photo of the suspects that the friends realized Tsarnaev may have been involved.

    The FBI claims this prompted Dias Kadyrbayev and Azamat Tazhayakova, both 19-year-old natives of Kazakhstan and friends of Tsarnaev at UMass-Dartmouth, to go to Tsarnaev's dorm and take the laptop, backpack and some Vaseline that may have been used in making the deadly pressure cooker bombs. Police believe the bombs were packed with shrapnel and gunpowder removed from fireworks.

    Robel Phillipos, of Cambridge, Mass., also 19, was charged with willfully making materially false statements to federal law enforcement officials during a terrorism investigation.

    Fox News' Catherine Herridge contributed to this report.