LOS ANGELES (AFP) - An American father charged with killing his two children with a spear fishing gun claims he was "enlightened" by QAnon and other conspiracy theories and was saving the world from "monsters" who had "serpent DNA", a federal complaint said.

Matthew Taylor Coleman, 40, told the authorities in California that he knew he was doing wrong, but that "it was the only course of action that would save the world", the complaint prepared by federal agents said.

Coleman was charged on Wednesday (Aug 11) with taking his children, aged two years and 10 months, to Mexico and killing them there before returning to the United States, where he was arrested.

The children's mother first raised the alarm on Aug 7 when Coleman took them from the home they shared. He had said he was taking them camping, but refused to tell her where and did not answer calls or text messages. He also did not have a child car seat with him.

A day later, police tracked him using the Find My iPhone app, which showed his last known location as being in Rosarito, Mexico.

When he crossed back into the US the next day, he was detained by the Federal Bureau of Investigation at the border.

Coleman confessed to shooting both his children with the spear fishing gun and leaving their bodies in Mexico, where they were found by the Mexican authorities.

He "stated that he believed his children were going to grow into monsters, so he had to kill them", according to the complaint.

He told agents that he was "enlightened by QAnon and Illuminati conspiracy theories and was receiving visions and signs revealing that his wife... possessed serpent DNA and had passed it on to his children", it continued.

He told federal law enforcement he believed he was "saving the world from monsters", the complaint said.

Coleman was charged with the foreign murder of US nationals.