Mohamad Jamal Khweis, 26, who grew up in Virginia, is the first ISIS-affiliated American to have been captured on the battlefield.
He appeared in court in Alexandria, Virginia, with family members and heard the charges he is facing. He didn't speak during the hearing and will be held at least until another hearing later this month.
Khweis' lawyer, John Zwerling, said after the hearing it was "too early to say" how his client would plead.
"Everything's not as it appears in the government pleading," Zwerling said, adding he just found out this morning his client was back in the U.S. and had been able to spend only an hour with him. "We will have our opportunity to explain the other side, at the appropriate time."
Khweis' father, Jamal, was with his son in court. "I'm just happy to see him today," he told reporters.
The judge ordered Khweis remain in detention until the next hearing on June 14.
After being captured by Kurdish anti-ISIS forces in March, Khweis said he renounced the terror group and described it as not representative of Islam, calling life under the organization "really, really bad" in a television interview.
Khweis has previously said he joined the terror group because of a woman.
The FBI interviewed Khweis while he was in Kurdish custody in Iraq, according to the criminal complaint filed against him.
The complaint alleges Khweis admitted to conducting "extensive research concerning ISIL" in 2015, including "frequently" watching ISIS execution videos and using social media to privately contact ISIS and smuggle himself across the Turkish-Syrian border.
He told the investigators, according to the complaint, that when an ISIS operative asked him whether he wanted to be a suicide bomber, Khweis said that he did. The complaint says Khweis thought that the "question was intended to test his commitment."
His lawyer said Thursday, "I have no reason to think he wanted to end his life for any particular reason."
After being captured by Kurdish anti-ISIS forces in March, Khweis told a local news organization that he had joined ISIS because of a young woman he met in Turkey who he said he accompanied into ISIS-controlled Syria.
"I wasn't thinking straight," he told Kurdistan24, adding that he left the terrorist group because he "wanted to go back to America."
Khweis told the Kurdish media outlet that he wanted to warn others against joining ISIS.
"My message to the American people is the life in Mosul, it's really, really bad," he said. "The people ... controlling Mosul don't represent the religion ... I don't see them as good Muslims."
"I didn't agree with their ideology, and that's when I wanted to escape," added Khweis.
More than 250 ISIS fighters in Syria are Americans, according to the U.S. director of National Intelligence. Khweis said he did not meet any other Americans while he was with the terror group.