Bryan LeBaron said American officials told the family that two suspects had been detained in the United States, but did not specify what role they had played in the Nov. 4 ambush attack.
Earlier this week, prosecutors said more than 40 suspects have now been identified in connection with the killings of the extended Langford and LeBaron families, who have lived in northern Mexico for decades. The family members suspect the shooting attack may have been a case of mistaken identity by drug cartel gunmen.
They consider themselves Mormon but are not affiliated with The Church of Jesus Christ of Latter-Day Saints. Many have dual U.S.-Mexican citizenship.
As many as 13 other members of La Mora - a decades-old settlement in Sonora state founded as part of an offshoot of The Church of Jesus Christ of Latter-day Saints - were missing after the attack on a convoy of three SUVs carrying community members, said a relative who asked not to be named for fear of reprisals.
The relative said he had located the burned-out, bullet-ridden SUV containing the remains of his nephew's wife and her four children - twin 6-month-old babies and two other children aged 8 and 10.
WATCH MORE: Relatives explain how oldest child hid kids behind tree during attack
The prosecutors' office did not offer any further details on the 40 suspects, many of whom are apparently known only by their nicknames.
Last week, prosecutors reported that three men were arrested and charged with organized crime for drug offenses, though none apparently yet faces homicide charges in the case.
They said four other suspects are being held under a form of house arrest. The name of one suspect announced by federal prosecutors Monday partially matches the police chief of the town of Janos, Chihuahua, near where the killings occurred. Local media reported the police chief had been in the pay of the La Linea drug gang.
Julian LeBaron, who lost relatives and friends in the ambush, confirmed the police chief had been arrested, and added, "That should be very worrying to everyone."
"Who vets them?" LeBaron asked. "He (the police chief) was there for 13 years", he said, questioning how state authorities could not have known the man was working for a drug cartel.
It would not be the first time that members of the break-away church had been attacked in northern Mexico, where their forebears settled - often in Chihuahua state - decades ago.
In 2009, Benjamin LeBaron, an anti-crime activist who was related to those killed in the attack, was murdered in the neighboring Chihuahua state.
The Associated Press contributed to this report.