Detroit, Michigan (CNN) -- Federal authorities decided to arrest members of the Hutaree militia group upon learning they were planning an exercise in April that might have included violence, a federal prosecutor said Tuesday.

"They talked about doing a covert reconnaissance exercise," said Barbara McQuade, U.S. attorney for the eastern district of Michigan, in an interview with CNN's Drew Griffin. "And they did also say that, in connection with the exercise, if they came upon anyone from the public or law enforcement who kind of stumbled upon them in the midst of this exercise, they were prepared to kill them."

That moved authorities to act, she said.

The militia members' ultimate plan, she said, was to call 911 reporting a fake emergency and then to kill the police officer who responded.Order.' "

The nine militia members charged in an indictment represent the core group, she said. Others participated in training but did not engage in criminal activity, she said.

Federal authorities on Monday charged nine Hutaree militia members in connection with the alleged plot. The group says on its Web site that Hutaree means "Christian warrior" and proclaims on its home page, "Preparing for the end time battles to keep the testimony of Jesus Christ alive."

But members of the group are outside the mainstream, a member of another militia group said Tuesday.

"This is a group that I would classify as neither a militia or a Christian group," said Michael Lackomar, a member of the Southeast Michigan Volunteer Militia. "They're more of a private army or a terrorist organization or really