He told authorities he became "better friends" with Tsarnaev, an ethnic Chechen, in spring 2012, and that he was a frequent visitor to the Tsarnaev home in Cambridge.
The FBI said that before Tsarnaev's roommate let the three friends into the room, Kadyrbayev received a text message from Tsarnaev that read: "I'm about to leave if you need something in my room take it," according to the FBI. When Tazhayakov learned of the message, "he believed he would never see Tsarnaev alive again," the FBI said in the affidavit.
It was unclear from the court papers whether authorities believe that was an instruction from Tsarnaev to destroy evidence.
Once inside Tsarnaev's room, the men watched a movie. At some point, they noticed a backpack containing more than a half-dozen fireworks, each about 8 inches long, according to the affidavit. The black powder had been scooped out.
The FBI said that Kadyrbayev knew when he saw the fireworks that Tsarnaev was involved in the bombings and decided to remove the backpack "to help his friend Tsarnaev avoid trouble."
Kadyrbayev also decided to remove Tsarnaev's laptop "because he did not want Tsarnaev's roommate to think he was stealing or behaving suspiciously by just taking the backpack," the FBI said.
After the three returned to Kadyrbayev's and Tazhayakov's apartment with the backpack and computer, they watched news reports featuring photographs of Tsarnaev. The FBI said 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, according to court papers.