Jerusalem — The arrest of a teenage Israeli-American for dozens of bomb threats against American Jewish institutions sent relief through Jewish communities on Thursday, but raised new questions about the threats and a broader rise in anti-Semitism.

Israeli police arrested the unidentified man — variously described as 18 or 19 years old — on suspicion of phoning in the bulk of the bomb threats, which have hit more than 100 Jewish centers in the United States since the beginning of the year.

In total, the Anti-Defamation League has counted more than 150 hoaxes, many which hit individual Jewish centers several times, causing shutdowns and evacuations of day schools. The man also is suspected of calling in a bomb threat against Delta Air Lines at New York’s John F. Kennedy airport in early 2015.

While it’s been praised by U.S. Jewish leaders, the arrest also has perplexed them. Local police said the man, who hid under his shirt when brought into a court south of Tel Aviv on Thursday, is Jewish and holds Israeli and U.S. citizenship.

His father also was arrested, and will be held for the next week, authorities said. His identity also is under a gag order.

“We are troubled to learn that the individual suspected of making these threats against Jewish Community Centers, which play a central role in the Jewish community, as well as serve as inclusive and welcoming places for all � is reportedly Jewish,” said Doron Krakow, CEO of the JCC Assn. of North America, in a statement. The group represents the majority of community centers that have been targeted.

Bomb threats are usually hoaxes, sometimes childish pranks and sometimes carried out to make a political or racist statement.

But, said Brian Levin, a hate crimes expert and professor at California State, San Bernardino, “It’s not always the stereotypical bigot. There are various types of offenders that commit these symbolic high profile acts, including the mentally unstable offender, those seeking personal benefit or revenge, thrill seekers, and those conflicted about their identity.”

He expressed concern that the arrest of a Jewish suspect would embolden the very people most likely to carry out anti-Semitic acts.

“Racists in the alt-right will play the ‘hate hoax’ angle of this from here on in,” Levin said.

Over the months, Jewish leaders have pressed President Donald Trump to speak out against the bomb threats, with many suggesting that the president’s campaign activities and his initial reluctance to talk about the crimes emboldened extremists.

During the campaign, Trump came under fire for retweeting white supremacists and anti-Semites, earning him support from the Ku Klux Klan and other hate groups. The president also was later criticized for purposely leaving any mention of Jews out of a statement observing the Holocaust and brushing off a question about anti-Semitism during a news conference as “insulting.”

Under pressure, Trump opened his first speech to a joint session of Congress by denouncing the “hate and evil” of the threats and recent vandalism of Jewish cemeteries.

Attorney General Jeff Sessions praised the FBI and the Israeli police for the arrest, describing it as the culmination of a large-scale investigation spanning multiple continents. “The Department of Justice is committed to protecting the civil rights of all Americans, and we will not tolerate the targeting of any community in this country on the basis of their religious beliefs,” Sessions said in a statement.