Those shirts have been distributed locally at community events, including National Night Out, Lt. Michael Berg of the Herndon Police Department said.
Law enforcement had not planned to arrest Duncan on Friday, according to an FBI agent's affidavit. But when they arrived at his front door with a search warrant, he ran barefoot out the back of the house and discarded a thumb drive that had been broken into pieces and immersed in a bubbling liquid, court files show.
Duncan had been on the FBI's radar since February 2016, according to the affidavit, when a relative reported that he had converted to Islam, expressed radical views and planned to travel to Turkey with his wife.
He and his wife, Zakiya Sadeq, 36, flew to Turkey that month, according to the affidavit, with tickets to go on to Bangladesh and then back to the United States. But they were denied entry into Turkey at the airport and returned home, the affidavit states.
From there, authorities learned that Duncan was communicating with overseas Islamic State supporters about joining the group and was searching online for weapons and body armor.
Sadeq was caught completely off guard by her husband's actions, her attorney, Faisal Gill, said. "She is as surprised as anyone — she's just shocked," Gill said Tuesday.
On the trip to Turkey, Gill said, Sadeq "had no intent of joining a terrorist organization. She was traveling through Turkey to go on her honeymoon."
The marriage caused friction with Duncan's family, his sister's fiancé told The Washington Post. At 36, Sadeq is considerably older than her husband and according to her LinkedIn profile worked as a doctor.
The couple lived in western Pennsylvania until June, when, according to the affidavit, their infant son died and they moved to Sterling, Va. Duncan had previously expressed interest in studying at an Islamic educational institute in Herndon, law enforcement officials allege. He faces up to 20 years in prison for obstruction of justice.
Duncan's public defender, Elizabeth Mullin, declined to comment after Tuesday's court appearance. He will appear in court again Monday for a detention and preliminary hearing.