In the months before that arrest, court filings allege, Sean Andrew Duncan, 21, who will appear in federal court on Tuesday, acted in a manner that was "indicative of an individual planning and researching how to conduct an attack," including research into materials relating to the Islamic State, terrorist attacks, weapons, surveillance tactics and body armor.
Now, family members are struggling to reconcile the man they had believed Duncan to be with the one federal agents say he had become.
"Sean is a very good kid. And that's all I can really say at this point. I'm floored," said his mother, Laurie Duncan, a real estate agent in Ocean City. "Sean is a very honest and sincere child, and that's all I can say, is that he's a child. He's 21."
Sean Duncan, who could not be reached for comment, grew up in eastern Baltimore County and, at first, was a "classic teenage boy," recalled Zach, who is engaged to Duncan's sister and requested that The Washington Post withhold his last name for fear of career repercussions.
As a boy, Duncan liked video games more than sports, didn't seem to have much ambition, and had trouble in school, Zach said. So much so that Duncan transferred from Patapsco High School to Patterson High School, where Zach said he befriended a few Muslim students and became interested in Islam.
"I accepted Islam during my last year of high school by the grace of Allah," as Duncan later described his "journey to Islam" in an online post he published to raise money for studying Koranic Arabic at Fawakih, an Islamic educational institute in Herndon, Va. "It was the summer before my 12th grade that I began looking into Islam, specifically how to pray the salah." He said he was struck by the "beauty" and "cleanliness" of the daily prayer and started listening to the Arabic recitation of the Koran, which, although he didn't understand Arabic, "penetrated my heart," causing him to fall "in love with the words of Allah."
He called the next chapter "Trouble on the Homefront."
"It was very hard for me to practice my Islam for at least the first 6-12 months," he wrote. "There is still some friction even until this day."
Some of the friction, Zach said, involved his relationship with a woman he met in his new social network, Zakiya Sadeq, now 36, who would become his wife. "She was in her 30s, and we were trying to figure out why this woman who is also studying medicine, how did she find this 18-year-old boy who doesn't have a job or money, and why is she interested in him?"
Duncan told the family he was going to marry Sadeq, who, according to her LinkedIn profile, obtained a medical degree at the International Islamic University Malaysia in Kuala Lumpur. "We were all like, 'What the hell are you doing?' " Zach said.
Through her attorney, Sadeq declined to comment.
The couple married, but in January 2016, according to court records, Duncan was also allegedly communicating with another woman whom the filing called an "unnamed co-conspirator." Duncan asked the woman to go to Syria with him and become his second wife, the filing said she told investigators. She "asked Duncan if his current wife would be okay with [her] coming with them to Syria," it said. "Duncan stated that his wife would have to be okay with it. [She] did not agree to go."
The next month, the FBI received a tip from one of Duncan's relatives saying he "may have been radicalized, and voiced his approval of Westerners being beheaded in the Middle East," according to court documents.
That year, Duncan, who had taken to wearing a religious robe and cap, and Sadeq moved to Pittsburgh, where Sadeq's LinkedIn profile says she worked as a doctor with the University of Pittsburgh Medical Center. They moved into a small apartment complex, as Duncan continued what was described in a Facebook post in September 2016 as his "journey to understand the Koran: the reason I became curious about Islam." He said he had just completed a Koranic Arabic course and planned to "teach this amazing language."
By the time of that posting, Sadeq was already pregnant with their child.
"They brought the baby down here," said the daughter of a neighbor who declined to give her name, calling Sadeq "very nice." "They weren't trouble or had any problems, but the baby died, and the next thing you know, they were gone."
On June 6 last year, 4-month-old Muhammad Duncan died of sudden unexplained infant death, according to the Allegheny County Medical Examiner's Office.
Soon after, they returned to the Washington region, where some family members on both sides are having difficulty comprehending the allegations against Duncan. "He seemed normal," said a cousin of Sadeq's, who asked not to be identified due to concern about repercussions.. "He seemed fine. They're both very simple people."