John Walker was the final prosecution witness. Lawyers for Tazhayakov, who is charged with helping Dhokhar Tsarnaev by hiding evidence, did not call any witnesses.
Tazhayakov, who was born in Kazakhstan and attended the University of Massachusetts Dartmouth with Tsarnaev, is the first person to go on trial in connection with the bombing. Tsarnaev could face the death penalty if he is convicted of setting off two bombs at the marathon finish line on April 15, 2013.
Walker said that during questioning he told Tazhayakov that Tsarnaev's life was "effectively over," but he could escape a long prison term by helping with the investigation. Tazhayakov faces up to 20 years in prison if he is convicted.
U.S. District Judge Douglas Woodlock scheduled closing statements for Wednesday.
Three people were killed and more than 200 injured in the bombing. Tsarnaev and his older brother, Tamerlan, allegedly killed a Massachusetts Institute of Technology police officer a few days later. Tamerlan was killed during a confrontation with police.
Dias Kadyrbayev and Robel Phillipos, both University of Massachusetts Dartmouth students, also face charges. Kadyrbayev allegedly joined Tazhayakov in removing Tsarnaev's backpack from his dorm room, while Phillipos, who also went to high school with Tsarnaev in Cambridge, Mass.,
allegedly lied to investigators.