(AP Photo/U.S. Marshall)

DETROIT - An undercover informant was paid about $31,000 in cash for his critical role in an investigation that led to charges against members of a militia accused of plotting rebellion against the U.S. government, an FBI agent testified Tuesday.

Agent Leslie Larsen was the first witness at the weeks-long trial of seven members of a southern Michigan militia called Hutaree. She answered a series of friendly questions from a federal prosecutor, mostly describing the timeline of the 2008-10 investigation and certain milestones in the case.

Larsen said informant Dan Murray secretly recorded conversations with the militia while he trained or attended meetings with members, especially leader David Stone. Murray received $25,000 for his time - 300 to 400 hours - and $5,600 for expenses, such as his cellphone, tent, travel and sleeping bags.

Murray will be a key witness later in the trial.

Militia members from Michigan, Indiana and Ohio are accused of conspiring to ambush and kill a police officer, then attack the funeral procession with explosives and trigger a broader revolt against the U.S. government. They deny the charges and claim authorities overreached.

"A new president comes into office. The agencies that are sworn to investigate and protect him are very, very curious about what's going on out there," defence attorney James Thomas told jurors in his opening statement. "That premise - that somebody would be out there who is going to be a danger, either to the country or to the president of the United States - got distorted.

"It was a conclusion that was brought to be supported by facts, facts that were manipulated," Thomas said.

Another defence attorney, Mark Satawa, signalled to the jury that Murray's credibility will be under intense scrutiny.

"There was not a single act of violence perpetrated by a single individual sitting over here. ... Don't let fear be the thing you fear," Satawa told jurors, a day after prosecutors during their opening statement held up weapons and military-style equipment seized during the 2010 arrests.

During her testimony, Larsen said Murray was paid $12,700 for work ahead of the Hutaree investigation. She said he attended meetings of at least one other militia but there were no indications of illegal activity.

Larsen said the FBI removed Murray from the Hutaree in January 2010, more than two months before charges were filed, because an undercover agent had been securely planted inside the group.

Murray had his own problems in February of that year when he fired a gun toward his wife and, in a separate incident, attempted suicide, according to court records. Defence lawyers have suggested that the informant's personal problems may have motivated the government to round up Hutaree members, not because of some threat against police as authorities have insisted.

Assistant U.S. Attorney Sheldon Light told the judge there's no evidence that the government intervened with local prosecutors to help Murray.

"Light's statement is accurate," said Maria Miller, a spokeswoman for the Wayne County prosecutor's office.

In court, the government played portions of videos apparently taken by Hutaree members and posted online. They show people dressed like soldiers firing weapons in the woods, crawling on their bellies and instructing one other about hand-to-hand combat. In one clip, a United Nations flag burns from a pole and is replaced by a Hutaree flag.

Larsen said she was concerned.

"Other militias have a designated training range, a designated firing range," the agent testified.

Of the original nine defendants, Joshua Clough is the only one to make a deal with prosecutors. He pleaded guilty in December to illegal use of a firearm and faces a mandatory five-year prison sentence. He could be called as a witness to testify for the government.

Besides conspiracy charges, all face at least one firearm charge.