For years, Abdullah el-Faisal, a Jamaican-born cleric, promoted an extreme form of Islam that advocated killing nonbelievers.
In Britain, his incitements sent him to prison in 2003. He was kicked out of Kenya in 2010. At home in Jamaica, he became an outspoken supporter of the Islamic State.
Now, he has been convicted in New York of supporting terrorism, attempting to support terrorism and conspiracy, offenses that took place without him setting foot in the city or planning an attack there.
During a trial that stretched over two months in State Supreme Court in Manhattan, prosecutors portrayed Mr. Faisal as a jihadist who had supported ISIS between 2014 and 2017 by spreading its propaganda, praising its grim theology as fighters bearing its black flag carried out a campaign of kidnapping, sexual slavery and murder across Iraq and Syria. He even acted as a marriage broker for the group, prosecutors said.
They added that Mr. Faisal had attempted to support ISIS through one specific matchmaking effort, conspiring with a young woman from New York who sought his assistance in entering territory controlled by the group and in marrying one of its fighters.
In fact, that woman was an undercover member of the New York Police Department’s clandestine Intelligence Bureau. She was one of three investigators who used fake names while communicating with Mr. Faisal from Manhattan, establishing jurisdiction there and gaining his trust while speaking the language of extremism.
Mr. Faisal was the first person to go to trial under New York State laws adopted a week after the 2001 destruction of the World Trade Center. Since then, officials with the Police Department and the district attorney’s office, who have seen Manhattan as a prime target for terrorists, have built teams to investigate and prosecute extremists.
On Thursday, Mr. Faisal, wearing a white shirt and white skullcap, listened quietly as the verdict was read and the jury polled. The five counts he faced carry potential sentences ranging from seven to 25 years in prison.
The trial provided an unusual window into how the Police Department’s secretive Intelligence Bureau, which has been criticized for spying on political protesters and American Muslims, pursued an international figure. It included days of testimony by a former intelligence detective and two others working for the bureau who were identified in court only by code names.
One traveled to Abu Dhabi, where Mr. Faisal put her in touch with an Islamic State fighter in Syria. Text messages introduced as evidence showed that the fighter, Luqmaan Patel, sent pictures of himself to the detective and asked how much she weighed. The detective wrote, “I want to join you guys,” and Mr. Patel said he would help.
Prosecutors said that Mr. Faisal had exhibited “a lifelong support of violent jihad,” but had a particular affinity for ISIS, which was known for drowning, burning and beheading prisoners.
“The defendant dedicated his life to ISIS,” a prosecutor, David Stuart, told jurors during his summation. “He worked tirelessly to support ISIS and ensure their success.”
Mr. Faisal’s lawyers described him as a blowhard who spouted noxious opinions. He had not recruited anyone to assist ISIS in a “specific act of terrorism,” one lawyer, Alex Grosshtern, told jurors. Another elicited testimony from the detective who traveled overseas in which she said she was not aware of Mr. Faisal causing physical harm to anybody.
That lawyer, Michael Fineman, also suggested that his client had been more interested in marrying the detective, identified in court as Undercover 716, than in introducing her to an ISIS fighter, adding that she had pushed that plan.
“She was taking all the steps,” Mr. Fineman said during his summation. “She did everything on her own.”
The prosecution’s case was built largely upon Mr. Faisal’s words over years, in lectures, emails, WhatsApp messages and Skype calls.
Prosecutors cited lectures in which Mr. Faisal had urged Muslims to engage in suicide bombings and encouraged them to attack nonbelievers “until you make the earth warm with their blood.”
They said that he had supported ISIS by disseminating the group’s messages. Evidence showed that Mr. Faisal also sent several people phone numbers he said were for “dawla,” an Arabic word that can mean state or dynasty and which prosecutors said he used to refer to ISIS.
Prosecutors added that Mr. Faisal had advised several ISIS fighters, justifying their actions, which according to a text message from Mr. Patel cited in court papers, included the beheading of Muslims who practice the Shia branch of Islam.
Text messages introduced as evidence show that in addition to communicating with ISIS fighters about tactics, Mr. Faisal had exchanges with some about marrying western women. And prosecutors said Mr. Faisal arranged or tried to arrange marriages between those fighters and women in Britain, Sweden and the United States.
The police investigation began in early 2016 when a male detective posing as a radical young Muslim woman from New York named Rojin Ahmed wrote to Mr. Faisal asking for his advice. Mr. Faisal suggested that she consider marrying his stepson, Hannibal Kokayi, adding “hijra is on his mind,” which prosecutors said meant he was thinking of traveling to join ISIS.
A female detective, referred to in court as Undercover 487, took over the role of Rojin and communicated for months with Mr. Kokayi, who appeared to hedge on any immediate plans to join ISIS. Finally, that detective told Mr. Faisal that she had traveled on her own to Jordan and he gave her a phone number for Mr. Patel.
Undercover 487 also introduced Mr. Faisal to the third detective, Undercover 716, who pretended to be a young woman named Mavish who was born in Pakistan and living in Brooklyn. Communications entered into evidence show that she told Mr. Faisal she planned to “make hijrah” and wanted a husband to help with that, asking about “the brother” he had mentioned to Rojin.
The investigation appeared to stall when instead of connecting the detective to Mr. Patel, Mr. Faisal urged her to fly to Jamaica and repeatedly offered himself as a romantic partner, writing: “I want you for myself.”
Undercover 716 testified that she did not accept, because she did not believe that Mr. Faisal would go to the Islamic State.
He had said several times that he did not like communicating with Undercover 716 while she was in the United States, believing she was being watched. So, in early 2017, the detective flew to Abu Dhabi.
She sent Mr. Faisal text messages from there in which she referred to joining dawla and wrote: “Please sheikh please help me,” and “I’m so close but just need a way to get inside.”
Mr. Faisal replied the next day with a phone number for Mr. Patel, writing: “Ring this brother.” When Undercover 716 asked if he was in Raqqa — a Syrian city that was then an ISIS stronghold — Mr. Faisal bristled.
“Don’t mention these things on my fone,” he replied. “I will get arrested.”