The verdicts against Steven William Carpenter, 56, Suzette Gulyas Gal, 55, Andras Zoltan Gal, 22, and Krisztian Zoltan George Gal, 29, followed a seven-day U.S. District Court trial before Judge Sam Haddon.
They were part of a scam that authorities said netted $673,000 from more than 50 investors, many of them elderly who were promised lucrative returns on fictitious oil and gas projects on Montana's Fort Peck Indian Reservation.
Carpenter, Suzette Gal and Andras Gal were found guilty on one count each of conspiracy to commit fraud, wire fraud and mail fraud. The jury found Krisztian Gal guilty of conspiracy to commit fraud but not guilty on two fraud charges.
Haddon ordered the four placed into custody, pending sentencing set for August 19.
Possible penalties in the case include up to 5 years in prison, a $250,000 fine and three years of supervised release on the conspiracy charges, and 20 years prison, a $250,000 fine and three years of supervised release for each of the wire and mail fraud charges.
The scam's ringleader, Mike Campa, pleaded guilty last week and awaits sentencing. A sixth defendant, Dana Yvonne Kent, was recently sentenced to 18 months prison under a plea deal.
Court documents show the government will seek $675,000 to $6 million in restitution.
Immediately after the guilty verdicts, federal prosecutors sought a restraining order against the defendants to prevent them from transferring their assets, including a bank account with more than $200,000 in it that is controlled by one or more of the Gals.
All of the defendants are from Southern California. Prosecutors said in court filings that Campa and Carpenter had a history of investment scams, including two prior convictions dating to the mid-1990s.
In court filings, prosecutors allege that several of the defendants lived "a life of luxury" with the proceeds from their scam, including trips to St. Thomas, Hawaii, the Dominican Republic and locations across the U.S. Prosecutors said none of the defendants had a legitimate employment that could have supported such a lifestyle.
U.S. Attorney Mike Cotter said in a statement that the verdict marked "a significant blow to the loose organization of professional telemarketers who prey every day on the country's elderly, desperate and gullible."
"The only way to stem the tide is for these predators to know, without question, that when they are caught the consequences will be significant," he said.
Cotter said hundreds of hours of undercover work went into the case, which was pursued by the FBI and the U.S. Department of Interior's Office of Inspector General.
The fictitious projects promoted by the defendants were linked to the booming Bakken oil patch of eastern Montana, with investors promised that their initial investments would be repaid within six months.
To make the story more believable, the defendants presented a copy of a 2006 letter from the federal Bureau of Indian Affairs approving three oil and gas leases on land owned by a Fort Peck tribal member. Authorities allege Campa and the others continued to solicit investors for another five years after the leases were canceled in 2007.
Suzette Gal is married to Campa, according to court documents. Andras and Krisztian Gal are her sons from a prior marriage.