WASHINGTON (AP) — U.S. authorities say an American-Israeli Jew charged with threatening to bomb Jewish community centers and schools across the United States advertised his threat-making services on an online black marketplace — and he may have had a customer.

Recently unsealed court documents link 18-year-old Michael Kadar to a posting on the now-shuttered illicit marketplace AlphaBay advertising a “School Email Bomb Threat Service.” The poster offered to send customized threats to schools for $30, plus a surcharge if the buyer sought to have someone framed. The price would increase if the buyer wanted to target more than one school or an entire district, according to the post, whose author offered refunds if a threat produced no evidence of success.

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Authorities say Kadar made 245 threatening calls, mostly to community centers and schools, from January to March, using an online calling service that disguised his voice and allowed him to hide his identity. The threats led to evacuations, sent a chill through Jewish communities and stoked fears of rising anti-Semitism.

He was arrested in Israel in March and has been charged in federal court in Orlando, Florida, with 28 counts of making threatening calls and conveying false information to police.

Investigators then sought permission to search several AlphaBay accounts they believed were being operated by Kadar, including the handle “Darknet_Legend,” according to the documents. On a thumb drive confiscated from Kadar’s home, investigators said they found a document that was nearly identical to an AlphaBay posting by “Darknet_Legend” offering bomb-threat services for sale.

Federal court records don’t list an attorney for Kadar in the U.S. But his lawyer in Jerusalem, Galit Bash, said shortly after his arrest that her client had a “very serious medical condition” that might have affected his behavior. She did not immediately return a call for comment Tuesday.

The AlphaBay advertisement described in court documents offers to customize the text of a bomb threat and frame someone for it, although it says “there is no guarantee that the police will question or arrest the framed person. I just add the persons name to the email.”

It wasn’t clear whether anyone actually paid Kadar for a threat, but “Darknet_Legend” received positive reviews from at least one other user. In a separate post, the user wrote: “Amazing on time and on target. We got evacuated and got the day cut short.” The review was posted the day after a California high school evacuated after receiving threatening emails.

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American and European law enforcement officials announced last month they had shuttered AlphaBay, which they called the world’s leading “darknet” marketplace that traded in illegal drugs, firearms and counterfeit goods.

AlphaBay went to great lengths to hide the identities of its vendors and customers, and it promoted money-laundering services to mask the flow of bitcoin and other digital currencies from investigators, authorities said.

But criminologist David Decary-Hetu, a darknet expert at the University of Montreal, said he’s seen no previous proven cases of such criminal services being sold on such online marketplaces.

“All the cases I have heard of so far turned out to be law enforcement trying to find people of interest,” said Decary-Hetu.

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Associated Press writers Ilan Ben Zion in Jerusalem and Frank Bajak in Houston contributed.