JERUSALEM — A monthslong wave of bomb threats against Jewish institutions in the United States that prompted evacuations, heightened security and fears of rising anti-Semitism gave way to an unexpected twist on Thursday. The person responsible for many of the threats, law enforcement officials said, was half a world away, in Israel, a Jewish teenager.
An intensive investigation spanning multiple countries culminated on Thursday in the arrest of the 18-year-old suspect, who holds dual Israeli and American citizenship, and his father. The teenager’s lawyer said he had a brain tumor that could affect his behavior.
The surge in threats over the past few months — well over 100 sent to Jewish community centers, schools and museums since the start of the year — coincided with an increase in hate crimes against a number of groups, from scrawled swastikas to homicides, feeding worries about a new era of bigotry. American and Israeli officials refused to say how many of those threats the suspect was accused of making. And some recent anti-Semitic acts were apparently committed by others, like threats against Jewish centers for which a Missouri man was charged, and the vandalizing ofJewish cemeteries.
But officials made it clear that they considered the teenager, who lived in the Ashkelon area of southern Israel, to be the primary source of the threats, though they did not offer a motive. “This is the guy we are talking about,” an Israeli police spokesman, Micky Rosenfeld, said.
The suspect made threats to sites in Australia and New Zealand, as well as in the United States, and to at least one commercial airline flight, prompting an emergency landing, Mr. Rosenfeld said.
A judge ordered the suspect, who has not been charged, held until at least March 30 and ordered a medical examination. His father was ordered held for eight days, on suspicion that he might have been aware of the threats, or even been involved.
The father denies any knowledge of the threats, his lawyer said. The judge imposed an order of silence forbidding the Israeli authorities to release either man’s name.
The teenager, who was born in Israel, has a brain tumor that can affect his cognitive abilities and lead to “irrational” behavior, his lawyer, Galit Bash, said. She would not say whether her client, who she said did not have a criminal record, had admitted or denied involvement.
Ms. Bash and the father’s lawyer, Eran Rau, who are both from the Office of the Israeli Public Defender, said the young man was an only child who lived with his parents and had been home-schooled, which is unusual in Israel. While most Israelis are drafted into military service, the teenager was rejected, which Ms. Bash said was because of his medical condition. Israeli news media reported that she had said in court that he had the tumor since he was 14.
The father, an engineer in his early 50s, was cooperating with investigators, Mr. Rau said. He said his client, who also has no criminal record, was concerned primarily with his son’s welfare, given his medical condition.
“This all seems very strange and preliminary to me,” Mr. Rau said.
Israeli news outlets reported that when the teenager was arrested, he tried to grab an officer’s gun. In his brief court appearance, the suspect, wearing khaki cargo pants, bowed his head and pulled up his shirt to conceal his face.
On Thursday morning, after months of investigation and waves of turmoil and panic, the Federal Bureau of Investigation held a conference call with leaders of Jewish organizations to discuss the surprising denouement to the investigation. As the news spread, it drew mixed reactions from Jewish leaders and anti-discrimination groups who tried to make sense of it.
Joel Dinkin, the executive vice president of the Evelyn Rubenstein Jewish Community Center of Houston, said that it was “a little bit perplexing from the standpoint of the fact that it’s somebody Jewish.”
Jonathan A. Greenblatt, chief executive of the Anti-Defamation League, cautioned that many of the people responsible for anti-Semitic threats, vandalism and “a torrent of abuse online” remained at large. And even the threats attributed to the Israeli teenager, he said, should still be considered acts of anti-Semitism.
“The motive may have been unclear, but the impact was crystal clear,” Mr. Greenblatt said. “These were acts that terrorized a community just because of their faith.”
Critics of President Trump have accused him of playing down hatred and violence against minority groups and charged that his anti-immigrant remarks are fueling conflict. He did not publicly condemn the anti-Semitic threats and vandalism until Feb. 21, weeks after Jewish groups began calling on him to speak out. On Feb. 28, in a meeting with state attorneys general, Mr. Trump suggested that some of the threats and vandalism could be a politically motivated effort to “make people look bad,” rather than actual expressions of bias, according to people who took part in the meeting.