SANTA ANA, Calif. — A federal judge on Monday sentenced a California man to 15 years in prison for trying to join the Islamic State group in Syria, calling his conduct a serious threat to the U.S. and saying it was influenced by his abusive childhood and history of mental problems.

Orange County native Adam Dandach, 22, was also ordered to undergo supervised release for the rest of his life after he pleaded guilty last year to trying to provide material support to a terror group and lying on his passport application.

The case against Dandach is one in a series involving young men in states ranging from California to Georgia who have been charged with trying to assist Islamic State and other groups.

The U.S. “faces significant threat from terrorists’ acts planned or committed by homegrown violent extremists like (the) defendant who become radicalized online and seek to engage in terror and support groups like ISIL,” federal prosecutors wrote in court filings before sentencing. ISIL and ISIS are acronyms for the Islamic State group.

The prosecutors had sought a 20-year sentence for Dandach, who authorities said had videos of executions and decapitations by terrorists stored on his computer. He also encouraged terrorist beliefs in online chatrooms, authorities said. “To this day he is making deliberate choices to continue to support this violent and horrific terrorist organization,” Celeste Corlett, an assistant U.S. attorney, said before Dandach’s sentencing.

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