Rep. Carolyn B. Maloney (D-N.Y.), center, a member of Congress’s bipartisan task force combating anti-Semitism, held a news conference March 3, 2017, at the Park East Synagogue in New York to address bomb threats against Jewish organizations and vandalism at Jewish cemeteries. (Bebeto Matthews/AP)

An 18-year-old with dual U.S. and Israeli citizenship was charged Friday with making threatening calls to Jewish community centers in Florida over three months, giving false information to police about harm to people in Georgia, and cyberstalking.

Michael Ron David Kadar, who lived in Ashkelon, Israel, at the time of the threats, took “extraordinary steps to conceal his identity and location” through technology, including voice alteration, use of proxy IP addresses, virtual currencies and caller ID “spoofing,” according to Acting Assistant Attorney General Kenneth E. Blanco of the Justice Department’s criminal division.

From Jan. 4 to March 7, Kadar allegedly made 245 threatening and graphic calls to numerous Jewish community centers, Jewish schools and Anti-Defamation League offices throughout Florida, indicating either that there were bombs in the buildings or that people were coming to commit mass shootings, according to the criminal complaint filed in U.S. District Court for the Middle District of Florida.

About noon on Jan. 5, for example, Kadar, whom the Israeli police identified as Jewish, allegedly called the Maimonides Hebrew Day School in Fort Myers, Fla., and said there was a “C-4 bomb” in the school and that shortly, shrapnel from the bomb would be directed into the heads of the Jewish children. He said during the call that the shrapnel would “blow their heads off,” according to the complaint.

No explosives were found, but many of the calls led to the temporary closure and evacuation or lockdown of the facilities that were targeted, and they forced law enforcement and other emergency personnel to respond and clear the areas.

“This kind of behavior is not a prank, and it isn’t harmless,” said FBI Director James B. Comey. “It’s a federal crime. It scares innocent people, disrupts entire communities and expends limited law enforcement resources.”

Federal prosecutors said that Kadar made the calls through online “spoofing services,” which allow a user to hide his true caller identification and disguise his voice. They say that he paid for the spoofing services with bitcoin, a type of digital virtual cash.

Kadar is accused of making 67 calls on Jan. 5 to facilities throughout the United States and abroad, using a spoofing service to disguise his voice to appear female. In one call to Orlando, he warned that there was going to be a “bloodbath” at a Jewish facility. In a call to a preschool in Tampa, he said two dozen children would be “slaughtered,” according to the complaint.

While Israeli authorities were conducting surveillance in Kadar’s neighborhood, they noticed a large parabolic antenna connected to his apartment unit. When they executed a search warrant, they found a flash drive with files about the spoofing company and the recorded calls. There was also a file titled “Bomb threats to Jewish Institutions.”

Kadar was also charged with making a phone call to a police department in Georgia and conveying false information about an alleged violent emergency at a private residence in Athens, Ga.

“The charges brought today demonstrate our resolve to pursue and prosecute those who seek to sow terror and fear in our community, wherever they may hide,” said Acting U.S. Attorney W. Stephen Muldrow, whose jurisdiction is the Middle District of Florida.” Justice Department officials said the investigation into threats of violence against Jewish institutions continues.