Mr. Tazhayakov was the first of four friends of the Tsarnaev brothers who faced charges stemming from the investigation. Mr. Kadyrbayev is set to be tried on the same charges in September.

Mr. Tazhayakov nodded and smiled tersely at his family as he entered the courtroom. His little sister, a toddler, was playing and grinning on the bench with their parents. His mother broke into sobs as the verdict was read. The jury deliberated for half of Wednesday, most of Thursday (before one juror became ill) and Monday morning. Mr. Tazhayakov faces a maximum of 20 years in prison on the obstruction charge and five on the conspiracy charge, though Judge Douglas P. Woodlock could consider lesser penalties at sentencing on Oct. 16. A lawyer for Mr. Tazhayakov, Matthew Myers, said he would appeal.

The six days of testimony in Boston’s federal court delved into the relaxed social dynamics of the three young men and their friends who played soccer video games and smoked marijuana, and offered glimpses of insight into Mr. Tsarnaev’s conduct before and after the bombings. A month before the marathon, prosecutors said, he discussed martyrdom and told his friends he knew how to make a bomb; in the hours after it, he apparently texted Mr. Tazhayakov, denying his involvement.

But the focus was on Mr. Tazhayakov. Prosecutors asserted that he was contacted by Mr. Kadyrbayev shortly after surveillance photos of the Tsarnaev brothers were released by the F.B.I. on April 18 — three days after the bombing. The government contended that the two met and looked at those photos on Mr. Kadyrbayev’s phone, and that Mr. Kadyrbayev showed Mr. Tazhayakov a text message from Mr. Tsarnaev imploring him to “go to my room and take what’s there.”