Story highlights The men purchased 18 rifles and handguns, a source says

The official said uncompleted pipe bombs were also found

They were comprised of store-bought materials

No sophisticated military-grade explosives were involved in the bombs construction

This much is clear: Four U.S. Army soldiers based in Georgia are accused of killing two people.

Beyond that, a Georgia prosecutor and federal authorities are offering differing responses to a possible plot by the group to overthrow the government and assassinate President Barack Obama.

"As far as the evidence has shown, the motive for the murders was the overthrow of the government," District Attorney Tom Durden said.

"This wasn't barroom talk," Durden said, describing the men as part of an anarchist militia. "They amassed a good bit of weapons and explosive materials."

A law enforcement official said the men had legally purchased at least 18 rifles and handguns in Washington and Georgia. The official said uncompleted pipe bombs were also found, and were comprised of store-bought materials. No sophisticated military grade-explosives were involved in their construction.

However, several agencies called into the investigation because of the accumulation of weapons -- including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives -- made scant mention of any alleged assassination plot or government overthrow attempt.

One official described it as a murder case and said no federal charges had been filed.

On Monday, Pfc. Michael Burnett laid out the elaborate plot, telling a southeast Georgia court that he was part of what prosecutors called "an anarchist group and militia."

Dressed in his Army uniform, he spoke in a Long County court about the group of Army soldiers and its role in the December deaths of former soldier Michael Roark and his teenage girlfriend, Tiffany York. Roark, he said, was killed because he took money from the group and planned to leave.

"I don't know how it got to the point where two people got murdered," Burnett said in court.

He talked about how he and three others accused -- Pvt. Isaac Aguigui, Sgt. Anthony Peden and Pvt. Christopher Salmon -- had begun getting together, "just going out shooting guns, just guy stuff."

"And then Aguigui introduced me to 'the manuscript,' that's what he called it, a book about true patriots," the soldier said.

The four men became part of a group that aimed "to give the government back to the people," according to Burnett, who said that revolution was its goal. They called it FEAR -- Forever Enduring Always Ready -- and spent thousands of dollars buying guns and bomb parts.

The government needed a change, Burnett told the court. "I thought we were the people who would be able to change it."

It is not clear how capable the group was of carrying out the goals Burnett laid out.

Assistant District Attorney Isabel Pauley said it was "unknown" how many others belonged to the group. She identified Aguigui as the leader of what she described as "an anarchist group and militia" that included active and former troops.

"Defendant Aguigui actively recruited new members at Fort Stewart (in southeast Georgia) and targeted soldiers who were in trouble or disillusioned," she said.

At the time of their arrest, group members had plotted a number of "acts of domestic terror," the prosecutor said.

These included "forcibly taking over the ammo control point of Fort Stewart to take the post, bombing vehicles of local and state judicial and political figureheads and federal representatives to include the local department of homeland security, (and plotting) to bomb the fountain at Forsyth Park in Savannah."

Days before he died, Roark had been discharged from the army, according to Pauley.

Roark and his 17-year-old girlfriend were killed because Aguigui felt the couple was "a loose end," Burnett said.

"Sir, if I could have stopped this from happening, I would have," the soldier told the judge about the couple's killings.

Burnett admitted being at the scene of the crime, including watching as a soldier "checked (York's) pulse and then shot her again."

York's sister, Tiffany, told CNN affiliate WTOC that she hoped York "didn't have to beg, or suffer."

As part of an agreement with prosecutors, Burnett pleaded guilty to manslaughter -- instead of murder, thus avoiding a possible death sentence -- and other charges. He also agreed to testify against the three other soldiers accused in the case.

All four soldiers had also been charged by the military in connection with the two killings. But as their case proceeded through civilian courts, the Army dismissed its charges, according to Fort Stewart spokesman Kevin Larson.

The military's Criminal Investigative Division (CID) probe is ongoing, though it is not believed there are any "unknown subjects" -- or people besides those four men -- tied to these crimes, Larson said.

In a statement Monday, Larson insisted that Fort Stewart and its affiliated Hunter Army Airfield do not have "a gang or militia problem."

"Any suspicions of gang activity are actively investigated by CID, (which) recognizes the obvious concerns with the combination of gangs and military-type training," he said. "That is why CID monitors and investigates gang and extremist group association with criminal acts in the Army so closely. We believe the reason we are able to maintain a low gang criminal threat status is because of the awareness of and focus on the threat."

Fort Stewart, about 40 miles southwest of Savannah, is home to the U.S. Army's 3rd Infantry Division.

Tens of thousands of troops, their dependents, civilian personnel and contractors live and work on the base, which encompasses 280,000 acres and includes parts of five counties, including Long County, which has about 14,500 residents. Hunter Army Airfield is in Savannah but is officially part of the larger Fort Stewart complex.

The Southern Poverty Law Center, which tracks what it characterizes as "hate groups" nationwide, spoke to Aguigui's father Monday night.