When the backpack was later found, inside it was a UMass-Dartmouth homework assignment sheet from a class Tsarnaev was taking, the FBI said.
The court papers do not say what happened to the laptop.
In a footnote, the FBI said: "Tazhayakov also informed the FBI agents that while eating a meal with Dzhokhar and Kadrybayev approximately one month prior to the marathon bombing, Dzhokhar had explained to Kadyrbayev and Tazhayakov that he knew how to make a bomb."
Robert Stahl, an attorney for Kadyrbayev, said his client "absolutely denies the charges" and didn't know that the backpack and fireworks were part of the bombing case. Kadyrbayev is "just as shocked and horrified by the violence in Boston that took place as the rest of the community is," the lawyer said.
He also denied that Kadyrbayev instantly recognized Tsarnaev's photo and said Kadyrbayev didn't know Tsarnaev was involved in the bombing: "His first inkling came much later," he said.
Tazhayakov's lawyer, Harlan Protass, said Tazhayakov "feels horrible and was shocked to hear that someone he knew at the University of Massachusetts Dartmouth was involved with the Boston Marathon bombing."
Kadyrbayev and Tazhayakov lived at an off-campus apartment in New Bedford, about 60 miles south of Boston, and got around in a car registered to Kadyrbayev with a souvenir plate that read "Terrorista (hash)1." The car was pictured on Tsarnaev's Twitter feed in March.
The plate was a gag gift from some of Kadyrbayev's friends, meant to invoke his penchant for late-night partying rather than his political sentiments, Kadyrbayev's lawyer said last week.
Breed reported from Raleigh, N.C. Contributing to this report were Associated Press writers Bridget Murphy and Rodrique Ngowi in Boston and Michelle R. Smith in Providence, R.I.