A northern Virginia man who became the subject of an FBI investigation after his family became suspicious of his conversion to Islam pleaded guilty Monday to charges including obstruction of a terror probe and receipt of child pornography.

Sean Duncan, 22, of Sterling entered the plea in U.S. District Court in Alexandria. The deal requires the judge to impose a prison term between 15 and 20 years.

Authorities arrested Duncan in December. When they arrived at his home to execute a search warrant, Duncan ran out the back door barefoot and tossed a plastic bag with a thumb drive broken into pieces and coated in a cleaning solution that produced frothing white bubbles.

The child pornography evidence emerged as the investigation continued.

Duncan did not plead to a terror-related charge in Monday's plea. But Duncan admitted as part of a plea that he had been espousing support for the Islamic State group as far back as 2015. At one point, he proposed marriage to a woman he had met online who was also an IS supporter. Duncan wanted the woman to be his second wife and meet up with him and his first wife in Syria. The woman declined.

In February 2016, Duncan and his wife flew from the U.S. for Turkey but were denied entry into that county.

By that point, according to court documents, one of Duncan's relatives had approached the FBI. The relative told the FBI that Duncan had converted to Islam and espoused radical views like support of beheadings.

Later, in October 2017, Duncan's name showed up on a list maintained by an Islamic State recruiter who had been arrested.

As for the child pornography, Duncan admitted that he had a smartphone that contained sexually explicit images of pre-pubescent minors, some as young as infants.

One of the photos showed Duncan's hand molesting an infant who was a relative.

Before Duncan moved to northern Virginia, he and his wife had been living in Aspinwall, a suburb of Pittsburgh. Duncan's infant son died in June 2017. The Allegheny County medical examiner's office listed the boy's manner of death as undetermined.

The photo showing the molestation of Duncan's infant relative was taken in the weeks and months after Duncan's son died.

Duncan, wearing a green jail jumpsuit and sporting a long beard, said little during Monday's plea hearing except to answer the judge's questions about whether he understood his rights.

The judge, Leonie Brinkema, is bound to impose a minimum of 15 years when Duncan is sentenced July 6. She can impose no more than 20 years.

The plea agreement does allow for the possibility that prosecutors can seek a reduced sentence if Duncan cooperates with authorities who are continuing their counterterrorism investigation.