The Philadelphia Inquirer reported 18-year-old Richard Tobin allegedly recruited people on a neo-Nazi social network.
Court records indicate Tobin told investigators he considered suicide attacks and once sat with a machete in the parking lot of a New Jersey mall, considering an attack on black shoppers.
An email message for his public defender wasn't returned.
The paper says the synagogues that were vandalized with swastikas and other images on Sept. 21-22 were in Hancock, Michigan, and Racine, Wisconsin.
A magistrate judge ordered Tobin to remain incarcerated Friday, pending a mental health evaluation.
The paper says authorities declined to discuss the details of their investigation.
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