The female relative, who didn't want to be named, called police in September 2011 to relay her worries about Isaac Aguigui, who is originally from the small town of Cashmere near Wenatchee, police Sgt. John Kruse said.
The woman told police that Aguigui's wife had died in July 2011 under suspicious circumstances, and the soldier had bought 15 firearms from a store in Wenatchee while on leave from the military.
Kruse said police checked the report and confirmed Aguigui had purchased 15 firearms, including some semi-automatic rifles, and did so legally.
As a precaution, police contacted the Army's criminal investigation division at Joint Base Lewis-McChord near Tacoma, which confirmed there was an open investigation regarding the wife's death, Kruse said.
Police also contacted the FBI's supervisory special agent Frank Harrill in Spokane. He declined to comment.
"It's my understanding that the FBI interviewed him," Kruse said of Aguigui.
Kruse believes Aguigui returned to Fort Stewart in Georgia shortly after buying the weapons.
Prosecutors in rural Long County, near Fort Stewart, said the militia group of active and former U.S. military members spent at least $87,000 buying guns and bomb components. They allege the group killed two people - former soldier Michael Roark and his 17-year-old girlfriend, Tiffany York - by shooting them in the woods last December in order to keep its plans secret.
One of the Fort Stewart soldiers charged in the case, Pfc. Michael Burnett, pleaded guilty Monday to manslaughter, illegal gang activity and other charges. He made a deal to cooperate with prosecutors against the three other soldiers.
Prosecutors said the group called itself F.E.A.R., short for Forever Enduring Always Ready.
Burnett said he knew the group's leaders by serving with them at Fort Stewart. He agreed to testify against fellow soldiers Aguigui, a private who was identified by prosecutors as the militia's founder and leader, and Sgt. Anthony Peden and Pvt. Christopher Salmon.
All are charged by state authorities in Georgia with malice murder, felony murder, criminal gang activity, aggravated assault and using a firearm while committing a felony. A hearing for the three soldiers was scheduled Thursday.
Aguigui's attorney, Daveniya Fisher, did not return a phone call seeking comment. Attorneys for Peden and Salmon both declined to comment.
Prosecutors say Roark, 19, served with the four defendants in the 4th Brigade Combat Team of the Army's 3rd Infantry Division and became involved with the militia. The group believed it had been betrayed by Roark - who left the Army two days before he was killed - and decided the ex-soldier and his girlfriend needed to be silenced, authorities said.
Prosecutors said Aguigui funded the militia using $500,000 in insurance and benefit payments from the death of his pregnant wife a year ago. Aguigui was not charged in his wife's death.
Prosecutors say the group plotted to take over Fort Stewart and to bomb a dam in Washington state and poison the state's apple crop. Authorities have said the militia's ultimate goal was to overthrow the government.
Tracy Jahr, Roark's mother, told KOMO-TV in Seattle: "I'm immensely proud of him for standing up for what he knew was right."
Jahr recalled a conversation with her son last fall, when he told her he had met a person with a lot of money.
"He died serving his country," Jahr told KOMO. "He wasn't in a war. He wasn't killed by the enemy. He was killed by his own people."