The death of a former neo-Nazi whose group patrols Arizona's desert near the Mexican border for illegal immigrants and drug smugglers is raising questions about his organization's future.

Friends of Jason Todd "JT" Ready vowed Friday that U.S. Border Guard's armed patrols will continue, but monitoring groups doubted the operations could be sustained.

Authorities say the 39-year-old Ready shot and killed his girlfriend and three others, including a toddler, before killing himself in a Phoenix suburb Wednesday, a murder-suicide stemming from domestic violence issues.

The Arizona Republic reported on its website Saturday that the FBI was already conducting a domestic terrorism investigation into Ready's activities prior to the shootings.

James Turgal, special agent in charge of the FBI's Phoenix office, told the paper that the FBI's investigation dated to when Ready was a member of the neo-Nazi National Socialist Movement and continued into his participation with the border group. The probe is based on tips of criminal activity that Turgal would not specify.

He stressed that it had nothing to do with this week's shootings.

Sean Rose, a 35-year-old Tucson man who said Ready was like a brother to him, said he would quit his job to keep the group going.

"He did a lot for this country as far as protecting the border, something the government doesn't do," Rose said. "I think it's good to have civilians stopping the drug market."

Groups that monitor the activities of organizations like the U.S. Border Guard expressed doubts that it will be able to maintain its operations. Without Ready's leadership, they say, the Border Guard will likely disappear.

"The U.S. Border Guard is probably finished," said Mark Potok of the anti-hate group, the Southern Poverty Law Center. "It really did revolve around JT Ready."

An SPLC recent report said that "nativist extremist" groups like Ready's decreased by almost half in 2011 to 184 groups, down from a high of 319 such groups in 2010.

The Minuteman Project and other similar groups have been plagued by infighting and financial difficulties, largely splintering or disintegrating altogether.

The movement's decline comes as states like Arizona passed harsh immigration laws that included provisions allowing local police to question a person's immigration status while enforcing other laws, Potok said.

Those laws created an impression among some civilian border militia members that state governments were doing more about illegal immigration, and that they no longer had to, he said.

Jennifer Allen, interim director the Arizona chapter of the immigrant advocacy group, the Southern Border Communities Coalition, said that organizations like Ready's thrive on a charismatic leader, and tend to implode once that leader is gone.

"What brings hate groups together is anger and fear. So it makes sense that they would start to direct that toward one another," she said. "They also attract a lot of people that want to be mega personalities.

"And it ends up being their own worst enemy -- fighting over the limelight," she said.

The SPLC also cites the case of Shawna Forde, a former member of the Minuteman Civil Defense Corps. She was convicted in a May 2009 home invasion that left a 9-year-old girl and her father dead.

Prosecutors said the invasion was an attempt to steal drug money to fund her group's border operations.

Forde was expelled from the Minuteman group in 2007 amid allegations of lying and pretending to be a senior leader. At the time of the killings, she was the head of her own group called the Minutemen American Defense.

The SPLC's report said the killings cast a pall over the entire civilian border militia movement.

But Ready began patrolling the desert with his group after the killings, dressing up in head-to-toe camouflage gear, helmets and boots, and carrying high-powered guns as they traveled out into the desert to look for illegal immigrants or smugglers.

Rose said that he never joined the group on patrols, but that he and Ready would go out with a handful of others about a dozen times a year on similar outings.

Rose and other friends of Ready's said they are reluctant to believe that he killed four people and himself, and say they feel drug cartels are more likely to blame even though police have discounted that possibility.

Police say all the evidence points to a domestic-violence situation.

Those killed in the rampage in Gilbert on Wednesday were Ready's girlfriend, her daughter; her 16-month-old granddaughter; and her daughter's boyfriend.

Harry Hughes, another close friend of Ready's and a regional director with the Detroit-based National Socialist Movement, said he plans to continue his own one-to-two man desert patrols.

Members of the National Socialist Movement promote white separatism, dress like Nazis and display swastikas. They believe only non-Jewish, white heterosexuals should be citizens and that anyone who isn't white should leave "peacefully or by force."

Ready was a former member of the group who said he quit to focus on his desert patrols.

"Just because Mr. Ready is no longer with us doesn't mean we're going to stop," Hughes said. "After we pay our last respects and get our ducks back in a row, I'm pretty sure business will continue."

He added, "I don't think JT would have wanted us to stop."