They were perhaps Dzhokhar Tsarnaev's closest friends during his two years at college: an American classmate from high school and two Russian-speaking students from Kazakhstan. The Kazakhs seemingly had money and drove expensive cars. They entertained Mr. Tsarnaev at their off-campus apartment, and he partied with them in New York. One of them lent Mr. Tsarnaev a black BMW after he smashed his Honda Civic in an accident.
And in the wake of the twin bombs that exploded last month at the finish line of the Boston Marathon, federal prosecutors now say, the three showed just how close their friendship was: Two of them decided to put a backpack and fireworks linking Mr. Tsarnaev to the blasts into a black trash bag and toss it into a Dumpster. Prosecutors say the third later lied to investigators when asked about it.
The two Kazakhs, Dias Kadyrbayev and Azamat Tazhayakov, were charged Wednesday with destroying evidence to obstruct the federal inquiry into the marathon bombing. Their American friend, Robel K. Phillipos, was charged with lying to impede the investigation.
The story behind their arrest, detailed in lengthy affidavits, paints a vivid portrait of Mr. Tsarnaev in the days after the bombing and portrays a dorm-room scene of confusion as the three young men, stunned to realize that their friend was being sought as a terrorist, debated whether and how to help him.
And it chillingly laid bare the skill with which Mr. Tsarnaev appears to have concealed plans for the bombing from even his most intimate associates. Three days after the blasts, as photographs of the then-unidentified bombing suspects blanketed television and the Internet, Mr. Kadyrbayev sent Mr. Tsarnaev a text message: One of the photographs, he wrote, bore a marked resemblance to him.
Mr. Tsarnaev coolly replied, "lol [netspeak for "laughing out loud"], you better not text me." He added: "Come to my room and take whatever you want."
Mr. Kadyrbayev told federal authorities that he thought the request was a joke. Only later that evening, he told interrogators, would he come to see it as a thinly veiled plea to cover up his crime.
Should the three men be found guilty, they would face potentially stiff penalties: as much as five years in prison for the two Kazakhs, eight years for Phillipos, and up to $250,000 fines for each of the three. Mr. Kadyrbayev, 19, and Mr. Tazhayakov, 20, have been held in jail since last week, ostensibly on suspicion of violating their student visas by not attending class at the University of Massachusetts at Dartmouth, where they had studied with Mr. Tsarnaev.
All four men entered classes there in the fall of 2011, but Mr. Phillipos dropped out and returned to Cambridge, where he and Mr. Tsarnaev had attended Cambridge Ringe and Latin High School together. A university spokesman said Mr. Kadyrbayev was not currently enrolled, and that Mr. Tazhayakov remained a student, but had been suspended until the charges against him are resolved.
In one respect, the two Kazakh students seem an odd match for Mr. Tsarnaev and Mr. Phillipos. Sent from oil-rich Kazakhstan to study in the United States, Mr. Tazhayakov and Mr. Kadyrbayev appear to have come from wealthy families. Mr. Kadyrbayev's Facebook page features photographs of him on beaches in Fort Lauderdale and Dubai. Mr. Tazhayakov's page indicates he comes from Atyrau, a petroleum center at the mouth of the Ural River. By contrast, the Cambridge homes of both Mr. Tsarnaev and Mr. Philippos are hard-worn apartment houses in working-class neighborhoods.
But the four quickly became close after starting classes, the affidavit and interviews with friends suggest, in part because Mr. Tsarnaev and the two Kazakh students all spoke fluent Russian. Mr. Tazhayakov struck up a friendship with Mr. Tsarnaev first and appeared the closest to him, said Jason Rowe, a sophomore who was Mr. Tsarnaev's freshman dorm roommate.
A Cambridge friend of Mr. Tsarnaev said their friendship began to ebb after Mr. Tsarnaev met the two Kazakhs. Photographs posted online suggest a deepening relationship with the foreign students; in one undated shot, Mr. Tsarnaev drapes an arm over a broadly smiling Mr. Kadyrbayev as the two sit at a kitchen table, plates of food laid out before them.
Despite dropping out of school and returning to Cambridge, Mr. Phillipos also appears to have become fast friends with the Kazakh students, visiting them frequently in the apartment they shared in New Bedford, about three miles from the Dartmouth campus. And Mr. Kadyrbayev and Mr. Tazhayakov apparently traveled often to Cambridge, Mr. Kadyrbayev to meet "repeatedly" with the Tsarnaev family, the criminal complaint against him states.
By last year, Mr. Tsarnaev and the two Kazakhs appear to have become constant companions. A 2012 photograph, possibly from last November, shows the three posing in Times Square, bundled against the cold, the Kazakh students grinning broadly.
An FBI affidavit supporting the charges against the three men does not detail their reactions to the bombings in Boston -- one of their lawyers said they had been "shocked and horrified" -- but it makes clear that for days afterward, they had no inkling that Mr. Tsarnaev might have been involved.
Wednesday, two days after the explosions, Mr. Kadyrbayev drove to Mr. Tsarnaev's dormitory and, standing outside, chatted while Mr. Tsarnaev smoked a cigarette, the affidavit quotes Mr. Kadyrbayev as saying. Later, Mr. Tsarnaev drove to the New Bedford apartment and stayed until about midnight.
Only one detail seemed amiss. Mr. Tsarnaev, whose long and unmanageable hair had been an object of wry posts on his Twitter account, had suddenly cut his mop short.
The next day, Mr. Tazhayakov told the FBI, Mr. Tsarnaev drove him home from a university class, dropping him off about 4 p.m. An hour or more later, Mr. Kadyrbayev called Mr. Phillipos as he was driving to the apartment from Boston with an urgent message: Turn on the television news when you get home.
Investigators had released grainy photographs of two bombing suspects, lifted from video surveillance cameras. One of the suspects, Mr. Phillipos said, looked familiar.
The sequence of events that followed, patched together from separate FBI interviews with Mr. Phillipos and the two Kazakhs, is not precisely clear. Sometimes before 7 p.m., the three men drove to Mr. Tsarnaev's Pine Dale Hall dormitory room. His roommate said Mr. Tsarnaev had left a couple of hours earlier.
As the visitors watched a movie, the affidavit states, they noticed a backpack stuffed with fireworks that had been emptied of their powder. Mr. Kadyrbayev "knew when he saw the empty fireworks that Tsarnaev was involved in the bombing," the affidavit states. He resolved to protect him. At 8:43 p.m., Mr. Kadyrbayev sent the text message to Mr. Tsarnaev, noting his resemblance to the photographs, and read the nonchalant reply.
Mr. Tazhayakov told the FBI that when Mr. Kadyrbayev showed him Mr. Tsarnaev's request to "take whatever you want," he concluded that he would never see his friend again alive.
At some point in the evening, the backpack, fireworks, a jar of Vaseline and Mr. Tsarnaev's laptop were carried back to the New Bedford apartment.
Mr. Phillipos initially told the FBI that he did not recall going to Mr. Tsarnaev's dorm room that night, then later said they had gone there but left without entering, authorities said. Only six days later would he recant: Actually, Mr. Kadyrbayev texted him at 9 p.m. to "go to Jahar's room," where the three men took the laptop and evidence.
Back home, Mr. Phillipos said, the three "started to freak out, because it became clear from a CNN report that we were watching that Jahar was one of the Boston Marathon bombers." Mr. Kadyrbayev asked him "if he should get rid of the stuff." Mr. Phillipos said he told him, "Do what you have to do."
Shortly thereafter, the bag and the fireworks were tossed into the apartment complex Dumpster. The next afternoon, as Mr. Tazhayakov watched, a garbage truck emptied it and drove away.