As part of the plan, 19-year-old Xavier Pelkey would rendezvous with underage accomplices from Chicago and Canada to stage an armed invasion of mosques that would include separating and killing adult men worshipers.
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The new guilty plea documents obtained by the I-Team reveal Pelkey had already written a public statement to be released when the Chicago attack began.
Pelkey now admits he had amassed and planned to use homemade explosives. Bombs and ISIS paraphernalia were part of a plot, according to Pelkey, targeting Shia Muslims in Chicago whom he and his unnamed terror accomplices considered non-believers in their more radical Sunni Islam world.
Weaponry was found in Pelkey's family home bedroom in rural Maine, more than 1,100 miles away from the Chicago targets.
The plot concocted by Pelkey used encrypted messaging applications, according to investigators.
RELATED: National security questions in case of Maine teen accused in plot against Chicago-area mosques
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The plea agreement with federal prosecutors in Maine comes less than five weeks before the teenager was to go on trial at the U.S. courthouse in Portland.
In it, he admits the Maine, Chicago and Canada conspiracy and having written drafts of a public statement that would be released just prior to the Chicago attack, proclaiming the name of ISIS, Jihad in America and striking fear in the hearts of Americans.
Former Chicago federal prosecutor Gil Soffer is ABC7's legal analyst. He explained how ISIS is able to recruit teenagers from all corners of the US.
"ISIS recruits teenagers in the same way that right, far-right wing, far-left wing and extremist organizations have recruited them for the last 20 years. They do so online. They do so with websites, they do so through social media," Soffer said. "Unfortunately, it's a very powerful tool, and this is yet further evidence of how successful it can be."
Soffer also warned that teenagers should be aware extremist conversations online may be monitored.
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"I don't think there's any question the message is, you may be watched, you may be talking to an undercover informant, you may be discovered in some other fashion. And if there's a conclusion that you're not just engaging in idle chatter, but that you mean to do something and in this case, the presence of these weapons certainly support the government's theory here, that you could face real consequences just as this defendant is facing," he said.
The plea hearing is a week from Friday and a sentencing date has to be set. The U.S. Attorney in Maine provided no comment.
Pelkey faces up to 15 years for supporting terrorism. His attorney has not responded to a request for comment.
One question hanging: what about the unnamed teenage co-conspirators in Chicago and Canada? Perhaps they had already begun cooperating, and that's why Pelkey pleaded guilty. But in any event, they have not been charged.