A California man confessed to killing his two young children, allegedly telling an FBI investigator he thought they were "going to grow into monsters so he had to kill them," according to a criminal complaint filed Wednesday. Matthew Taylor Coleman, a 40-year-old Santa Barbara surfing school instructor, has been charged with foreign murder of US nationals after allegedly taking his 2-year-old son and 10-month-old daughter into Mexico and killing them, according to an affidavit filed as part of a federal criminal complaint and a statement from the US Attorney's Office for the Central District of California. Stopped for questioning at the US-Mexico border, Coleman allegedly told investigators he shot the children with a spear fishing gun, then hid their bodies, the affidavit said. Coleman spoke to authorities about being "enlightened by QAnon and Illuminati conspiracy theories," including that he had visions that his wife "possessed serpent DNA and had passed it onto his children," according to the affidavit. Coleman was detained at the border and transported to a local jail by authorities. CNN has reached out to Coleman's lawyer but has not received a response. He was scheduled for an initial court appearance Wednesday in US District Court in Los Angeles, the US Attorney's Office said.