(Reuters) - A federal grand jury has handed down new child sexual exploitation charges against a California man who investigators now suspect obtained sexually explicit images of more than 300 underage victims from the United States and worldwide.
Blake Robert Johnston, 41, of Martinez, was charged in October 2014 with traveling to another state with intent to engage in illicit sexual conduct with a minor, the U.S. Immigration and Customs Enforcement (ICE) said on Monday.
New charges lodged against him last week by a grand jury in Oakland include transporting a minor with intent to engage in criminal sexual activity, online coercion, sexual exploitation involving a minor, and production of child pornography, it said.
A study of Johnston's computers and mobile devices led to the identification of 31 possible underage victims in 12 states and overseas, ICE said in a statement, including three underage girls who allegedly had physical contact with him.
"Based on the analysis ... investigators now estimate there could be more than 300 other potential underage victims in this case, both domestically and worldwide," it said.
ICE said it is working with authorities in foreign countries including Australia, Ireland and the United Kingdom.
"We're appealing to anyone who has awareness about this defendant's activities or his past contacts to come forward," Tatum King, acting special agent in charge for Homeland Security Investigations in San Francisco, said in the statement.
Investigators believe Johnston used multiple mobile applications to communicate with underage victims, including Kik, Skype, Omegle, ooVoo and Facebook, ICE said.
He has been held without bond in federal custody since last October, it added, and is due in court again on Tuesday.
(Reporting by Daniel Wallis in Denver; Editing by Doina Chiacu)