(CNN) — A series of encrypted phone calls – secretly recorded by a confidential FBI source – tipped off federal agents that a group was allegedly planning to carry out a terrorist attack in Michigan on behalf of ISIS, according to a criminal complaint unsealed Monday.

Mohmed Ali, 20, and Majed Mahmoud, 20, face charges of “Receiving and transferring, and attempting and conspiring to transfer, firearms and ammunition knowing and having reasonable cause to believe that the firearms and ammunition would be used to commit a Federal crime of terrorism,” according to the criminal complaint.

Federal officials have yet to charge a third person – a juvenile described throughout the complaint as Person 1 – in connection with the alleged plot.

But the complaint details how agents doggedly surve

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