Bilal Abood, 37, is an Iraqi-born naturalized U.S. citizen who came to the United States in 2009.
Abood made his initial appearance in federal court in Dallas on Tuesday and will remain in custody pending a probable cause and detention hearing set for Friday.
The intelligence community recently warned of an uptick in ISIS recruitment, and the FBI has had its eye on Abood, who migrated to Texas in 2009.
FBI officials said Abood pledged an oath to Abu Bakr al-Baghdadi, the leader of ISIS, on June 19, 2014.
Abood, according to a search warrant, had been viewing ISIS atrocities online and had used his Twitter account to tweet and retweet information on al-Baghdadi.
Abood had also traveled to the Middle East in 2013, leaving through Mexico and eventually getting to Syria via Turkey. Abood told investigators he went to Syria to fight the Assad regime with a group called the Free Syrian Army.
Abood told investigators he had not financially supported ISIS or any other terrorist group.
Abood's maximum punishment is eight years in federal prison and a $250,000 fine.
The FBI says that Abood spent time at his Mesquite apartment watching propaganda videos of ISIS, including beheadings.
But the girlfriend he moved in with in February tells FOX 4 that he only used his computer to play videogames.
"I think it's B.S.," Abood's girlfriend, Barbara Strebeck, told FOX 4 by phone. "I think it's a bunch of craziness because of the fact that he didn't lie to no one. He told the truth."
Strebeck is currently recovering from surgery. She denies the FBI's allegations.
"Does he have any involvement with ISIS? Has he ever mentioned ISIS at all to you?" asked FOX 4's Zahid Arab.
"No," said Strebeck.
Strebeck claims Abood's arrest is the government retaliating against him.
"Because he would not spy for them," she said. "They're doing this on purpose. He didn't lie. He told the truth."
FOX 4 asked Strebeck for a picture of Abood, but she says he deleted most of them because of fear his family may find out about their relationship –- Abood is Muslim and Strebeck is not.
Abood's neighbor, Crystal Lewis, lives two doors down from him at the Spanish Meadows apartments in Mesquite. She says something seemed off about him.
"He wouldn't show his face," she said.
Abood was taken into custody around 6:15 a.m. Thursday.