From the story here:
... Like
other terrorism prosecutions, this case is expected to focus on
protected free speech, with one side arguing the defendants were merely
expressing their views, while the other maintains they were planning to
do harm. So far, the government has not produced any anti-gay text
messages, social media comments or conversations by the defendants,
whose communications and movements were being monitored by undercover
informants and agents. ...
The FBI, which started investigating the Dearborn men a year ago, used
undercover informants and agents to monitor the defendants'
communications. ...
To
date, the government has charged eight individuals in this case: five
from Michigan, including two juveniles who are being prosecuted in a
confidential process because of their age; two from New Jersey,
including one man who allegedly referred to himself as the biggest anti
Semite in America, and said he wanted to kill his mother's Jewish
friends; and one man from Washington, whose plans to travel overseas to
join ISIS were cut short by the FBI raids in Michigan. He was arrested
at his home in Seattle, one day before his plane was to leave, court
records show.
The
charging documents filed in all eight cases, so far, do not show how
the New Jersey or Washington defendants were allegedly tied to the
purported Michigan Halloween terror plot.
It sounds like the FBI worked pretty hard on these young fellas for a long time to get the statements they needed.