The defendants -- Guled Ali Omar, 21; Abdirahman Yasin Daud, 22; and Mohamed Abdihamid Farah, 22 -- showed little emotion as the three-week trial came to a close. Several people sitting in seats reserved for family broke down in tears; others left the courtroom in disbelief.
US District Judge Michael Davis didn't immediately set a sentencing date, saying he wanted to review the case and hear directly from the Somali-American men before making a decision.
"Can I write to you?" Omar quietly asked the judge.
"Yes sir, you can write to me anytime," Davis responded. Looking at Farah, he said: "And this time, I don't want you to hold anything back."
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Young men from Minnesota's Somali community, the nation's largest, have been a target for terror recruiters in recent years.
Prosecutors have said Omar, Daud and Farah were part of a group of friends who inspired and recruited each other to join the Islamic State organisation. Six others earlier pleaded guilty to one count each of conspiring to support a foreign terrorist organisation. A seventh, 22-year-old Abdi Nur, is at large, believed to be in Syria.
"These were not wayward kids who just got caught up in a fantasy," said US Attorney Andrew Luger, adding that he was satisfied with the federal jury's ruling. "They made a deeply personal and deliberate decision back in 2014. They wanted to fight for a brutal terrorist organisation, kill innocent people and destroy their own families in the process."
He called the trial "one of the most important" seen in Minnesota in years, because it put a spotlight on ongoing terror recruitment.
The three defendants, all from Minneapolis and arrested a year ago, pleaded not guilty during the trial.