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.
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.
Sen. Chuck Grassley, R-Iowa, in a three-page letter sent to Department of Homeland Security Secretary Janet Napolitano Thursday, asked for additional details about the student visa applications for  Tazhayakov and Kadyrbayev, and how Tazhayakov was allowed to re-enter the United States in January.
The lawyers for the Kazakh students said their clients had nothing to do with the bombing and were just as shocked by it as everyone else. Phillipos' attorney said the only allegation against him is "he made a misrepresentation."
Meanwhile, the body of Boston Marathon bombing suspect Tamerlan Tsarnaev was claimed Thursday evening.
A representative of the Dyer-Lake Funeral Home in North Attleboro, Mass., confirmed to The Sun Chronicle that Tsarnaev's body was brought to the funeral home from the Boston Medical Examiner's office by one of its hearses.
Local Boston station WFXT reported that a hearse believed to be carrying Tsarnaev's body was escorted away from the funeral home by police escort late Thursday. Its destination was not immediately known.
Local police were dispatched to the funeral home where media gathered. According to The Sun Chronicle, it is unclear why the funeral home was involved.
Department of Public Safety spokesman Terrel Harris said a funeral home retained by Tsarnaev's family picked up the 26-year-old's remains. He had no more information.
The medical examiner determined Tsarnaev's cause of death on Monday, but officials said it wouldn't become public until his remains were released and a death certificate was filed. It was unclear on Thursday evening whether the death certificate had been filed.
Fox News' Catherine Herridge and The Associated Press contributed to this report.