ALEXANDRIA, Va. — The FBI first focused on 21-year-old Sean Duncan when family members reported that he had converted to Islam and may have radicalized.

The bureau’s interest deepened after his name showed up on a 10-page list of people who had contacted a suspected recruiter for the Islamic State.

Those suspicions boiled over to an arrest when the bureau executed a search warrant Friday at Duncan’s northern Virginia home. According to an FBI affidavit, agents knocked on Duncan’s door to execute the warrant but no one answered.

Instead, Duncan ran out the back door, barefoot. Agents ordered Duncan to stop, but before he did, he tossed a clear plastic bag over the agents’ heads. Inside the bag, according to an affidavit, was “a thumb drive that had been snapped into pieces, and placed in a liquid substance that produced frothy white bubbles.”

Now Duncan is charged with obstruction of justice for the alleged destruction of evidence in a terrorism investigation, carrying a potential 20-year prison sentence.

Duncan made a brief initial appearance Tuesday in U.S. District Court in Alexandria, wearing a black T-shirt with a U.S. flag on the back saying “Herndon Police Supporter.” A public defender was appointed to represent Duncan. A magistrate ordered that Duncan remain jailed until a detention hearing, which was scheduled for Monday at the defense’s request.

The public defender, Elizabeth Mullin, declined comment after Tuesday’s hearing.

It is not known whether prosecutors plan to bring additional charges against Duncan. Joshua Stueve, a spokesman for the U.S. Attorney’s Office for the Eastern District of Virginia, which is prosecuting the case, declined to speculate Tuesday on additional charges but acknowledged that the affidavit includes details linking Duncan to what is “certainly an ISIS-related terrorism investigation.”

The affidavit says Duncan has been under FBI scrutiny since February 2016, when he was living in Aspinwall, Pennsylvania, a Pittsburgh suburb. Family members told authorities that Duncan had converted to Islam and may have radicalized, according to the affidavit.

Late that month, Duncan and his wife flew to Turkey, with a connecting flight to Bangladesh. But Turkish authorities denied the couple entry into the country, and they were sent back to the U.S.

In other terror cases, authorities have said that people looking to join the Islamic State often try to do so through Turkey.

The affidavit also spells out contact between Duncan and a woman who was arrested in a foreign country for planning to join the Islamic State. According to the affidavit, Duncan sought to make her his second wife and travel to Syria with her. The woman declined his proposal.

The affidavit also states that Duncan’s name, listed as “Sean Ibn Gary Duncan,” showed up on a 10-page list of people who had reached out to an Islamic State recruiter in another foreign country.

Lastly, police in the Pittsburgh area provided the FBI evidence taken from Duncan’s phone in June 2017, when police there were investigating the death of Duncan’s 4-month-old son, Muhammad. A review of the phone found numerous searches related to the Islamic State, weapons, body armor and hidden cameras.

An autopsy conducted by the Allegheny County medical examiner listed the cause of the baby’s death as “sudden unexplained infant death.”