Three months before allegedly killing his 2-year-old son and 10-month-old daughter, Matthew Taylor Coleman shared an oceanside family photo on Instagram

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California surf instructor Matthew Taylor Coleman — who is accused of killing his two children with a spearfishing gun — described his family as "treasures" in the last photo he shared of them all together.

On May 6, Coleman, 40, shared a photo on Instagram with his wife and children Kaleo, 2, and Roxy, 10 months, standing on the beach, smiling for the camera.

"My treasures!" he wrote in the caption.

Just a couple of days later, on May 11, he shared another photo of the toddler and baby snuggling together outside.

On Monday, three months after sharing the touching photos, Coleman allegedly killed his two children in Mexico because he believed they "were going to grow into monsters," according to a criminal complaint filed in the U.S. District Court obtained by PEOPLE.

Matthew Taylor Coleman
Credit: Matthew Taylor Coleman/instagram

Coleman claimed to be "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," FBI special agent Jennifer Bannon wrote in the 10-page complaint.

"[Coleman] stated that he believed his children were going to grow into monsters so he had to kill them," Bannon wrote.

Matthew Taylor Coleman
Credit: Matthew Taylor Coleman/instagram

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Coleman traveled with his children to the border vacation town of Rosarito and checked into a hotel on Saturday, police say, and the children's mother reported them all missing on Sunday. He then took the baby and toddler to a ranch early Monday morning, where he killed them, investigators believe.

"Coleman stated that he knew it was wrong," Bannon wrote, "but it was the only course of action that would save the world."

Agents with Baja California's State Security and Investigation Guard contacted U.S. authorities after the children's bodies were discovered, and Coleman was stopped by American border officers at the San Ysidro Port of Entry.

Coleman was arrested and charged two days later with the foreign murder of U.S. nationals.

Coleman, who has not yet entered a plea, remains in federal custody without bond and will be arraigned on August 31.