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A look at the basics of the investigation into the April 15 Boston Marathon bombing:
Three college friends of suspected bomber Dzhokhar Tsarnaev were arrested Wednesday for allegedly removing a backpack containing hollowed-out fireworks from Tsarnaev's room three days after the bombing. Authorities said the trio of 19-year-olds concluded their friend was involved in the bombing after seeing news reports that included pictures of the suspects. Two students from Kazakhstan were charged with conspiring to obstruct justice by concealing and destroying evidence, while the third student was charged with lying to investigators.
The FBI said that about a month before the bombing, Tsarnaev had told two of the men he knew how to make a bomb.
THE NEW SUSPECTS
Azamat Tazhayakov and Dias Kadyrbayev, both from Kazakhstan, have been held in jail for more than a week on allegations they violated their student visas by not regularly going to class at the University of Massachusetts at Dartmouth. At a court appearance Wednesday, they did not request bail and will be held for another hearing May 14. Attorneys for both men said their clients had nothing to do with the bombing and were shocked by the crime. Kadyrbayev's car had a souvenir license plate that read "Terrorista (hash)1" that his attorney said was a gag gift about his penchant for partying.
Robel Phillipos graduated from the prestigious Cambridge Rindge and Latin High School with Tsarnaev. He was held for a hearing scheduled for Monday.
Authorities say brothers Dzhokhar and Tamerlan Tsarnaev, ethnic Chechens from Russia, fashioned two bombs out of ordinary kitchen pressure cookers and set them off about 10 seconds apart near the finish line of the Boston Marathon. The explosions ripped off limbs and sent shrapnel flying, killing three people and wounding more than 260. Investigators believe the brothers were motivated by a radical brand of Islam, although they do not seem connected to any Muslim terrorist groups.
The explosions touched off a massive manhunt after authorities released the pictures of the suspected bombers. Authorities said the brothers shot and killed a Massachusetts Institute of Technology police officer, carjacked an SUV and engaged in a gunbattle with police in the streets of the suburban Boston community of Watertown. Tamerlan Tsarnaev was wounded during the firefight, and investigators said Dzhokhar Tsarnaev ran over him in the stolen SUV as he made his escape. Tamerlan Tsarnaev was later pronounced dead at a hospital.
Authorities shut down Boston and other nearby communities during their daylong search for the younger brother. Dzhokhar Tsarnaev was found hours later, badly wounded and hiding under a tarp covering a pleasure boat in the backyard of a Watertown home. He's currently being held in a prison hospital and is charged with using a weapon of mass destruction to kill, a crime that carries a potential death sentence.