A California surfing instructor confessed to killing his two children with a spearfishing gun after abducting them, telling investigators that his belief in the conspiracy theories known as QAnon made him do it, the authorities said on Wednesday.
The father, Matthew Taylor Coleman, of Santa Barbara, drove his 2-year-old-son and 10-month-old daughter to Mexico over the weekend and fatally shot them on Monday, according to an F.B.I. investigator.
In a nine-page criminal affidavit that was filed in U.S. District Court in Los Angeles on Wednesday, the F.B.I. special agent wrote that Mr. Coleman told the authorities that he knew what he did was wrong, but that it was the only course of action that would save the world.
“M. Coleman stated that he believed his children were going to grow into monsters so he had to kill them,” the special agent, Jennifer Bannon, wrote. “M. Coleman explained that he was enlightened by QAnon and Illuminati conspiracy theories and was receiving visions and signs revealing that his wife, A.C., possessed serpent DNA and had passed it on to his children.”
Mr. Coleman described himself as an adherent of QAnon, the pro-Trump conspiracy movement whose followers hold, falsely, that a cabal of Democrats and coastal elites engage in pedophilia and worship Satan. The conspiracy movement has been the subject of warnings by the F.B.I. that its followers could resort to violence.
Mr. Coleman, 40, was charged on Wednesday with foreign murder of U.S. nationals and remains in federal custody, the authorities said.
A federal public defender who represented Mr. Coleman at a detention hearing on Wednesday in federal court in Los Angeles, where he was ordered held without bond, did not immediately respond to a request for comment. Mr. Coleman is scheduled for arraignment on Aug. 31.
Mr. Coleman is the founder of Lovewater, a surfing school in Santa Barbara that offers private and group lessons, in addition to surf therapy. A biography on the company’s website described him as a champion surfer and youth mentor who enjoys spearfishing and sailing. The website features photos of Mr. Coleman with his wife and two children.
Investigators said that Mr. Coleman’s wife first contacted the Santa Barbara Police Department on Saturday, the same day that Mr. Coleman drove off with the couple’s children in their Mercedes Sprinter van and did not say where he was going. The family was supposed to go camping, and Mr. Coleman did not respond to his wife’s text messages, the authorities said.
Using the “find my phone” function on her laptop, Mr. Coleman’s wife learned on Sunday that he was in Rosarito, Mexico, a beach town about 16 miles south of the border city of Tijuana, according to the criminal affidavit.
On Monday, it showed that Mr. Coleman was still in Mexico and was near the San Ysidro Port of Entry, where another F.B.I. agent interviewed him when he attempted to cross back into the United States, the authorities said. There was no sign of his children, according to investigators, who said that they had found blood on the vehicle’s registration papers.
The bodies of Mr. Coleman’s two children — identified in court records only by their initials: The girl was R.C. and the boy was K.C. — were found in a ditch by Mexican authorities, and they had large puncture wounds to their chest cavities, the F.B.I. said. In the affidavit, investigators wrote that Mr. Coleman told them that he had discarded the spearfishing gun in a creek and his bloody clothes in a trash bin at the side of a road in Tijuana.
Investigators said that Mr. Coleman told them that his son was still alive after he shot him with the spearfishing gun and that he continued to move the spear around. He told an F.B.I. agent that he had fired the spearfishing gun into his daughter’s heart, the authorities said.