The affidavit says Tazhayakov and Kadyrbayev agreed to get rid of the backpack after concluding from news reports that Dzhokhar Tsarnaev was one of the bombers. A court appearance for the three is scheduled for Wednesday afternoon.
Three people were killed and more than 260 injured on April 15 when two bombs exploded near the finish line. The suspect's brother, Tamerlan Tsarnaev, died after a gunfight with police several days later. Dzhokhar Tsarnaev was captured and lies in a prison hospital.
Tazhayakov and Kadyrbayev have been held in jail for more than a week on allegations that they violated their student visas while attending UMass. Linda Cristello, the Boston attorney who represented them at a hearing on the immigration case Wednesday morning, confirmed earlier that they were facing separate federal charges.
All three men charged Wednesday began attending UMass with Tsarnaev at the same time in 2011, the FBI affidavit says.
Authorities allege that on the night of April 18, after the FBI released photos of the bombing suspects and the three men suspected their friend was one of them, they went to Tsarnaev's dorm room. The men noticed a backpack containing fireworks, which had been opened and emptied of powder.
The FBI said that Kadyrbayev knew when he saw the empty fireworks that Tsarnaev was involved in the bombings and decided to remove the backpack from the room "in order to help his friend Tsarnaev avoid trouble." He also decided to remove Tsarnaev's laptop, the FBI said in the affidavit.
After the three men returned to Kadyrbayev's and Tazhayakov's apartment with the backpack and computer, they watched news reports featuring photographs of Tsarnaev. The affidavit says Kadyrbayev told authorities the three men then "collectively decided to throw the backpack and fireworks into the trash because they did not want Tsarnaev to get into trouble."
Kadyrbayev said he placed the backpack and fireworks along with trash from the apartment into a large trash bag and threw it into a garbage bin near the men's apartment.
Meanwhile, Tamerlan Tsarnaev's relatives will claim his body now that his wife has agreed to release it, an uncle said. The body of Tsarnaev, 26, has been at the medical examiner's office in Massachusetts since he died after a gunfight with authorities more than a week ago.
Amato DeLuca, the Rhode Island attorney for his widow, Katherine Russell, said Tuesday that his client had just learned that the medical examiner was ready to release Tsarnaev's body and that she wants it released to his side of the family.
Police said Tsarnaev ran out of ammunition before his 19-year-old brother dragged his body under a vehicle while fleeing the scene. His cause of death has been determined but will not be made public until his remains are claimed.
"Of course, family members will take possession of the body," uncle Ruslan Tsarni of Maryland said Tuesday night. "We'll do it. We will do it. A family is a family."
He would not elaborate. Tsarnaev's parents are still in Russia, but he has other relatives on his side of the family in the U.S., including Tsarni.
Contributing to this report were Associated Press writers Michelle R. Smith in Providence; Rodrique Ngowi in Boston; Lynn Berry in Moscow; Arsen Mollyaev in Makhachkala, Russia; and Eric Tucker, Alicia A. Caldwell, Eileen Sullivan and AP Intelligence Writer Kimberly Dozier in Washington.
Copyright 2013 The Associated Press.