A U.S. federal judge in the city of Asheville, North Carolina, ordered the indefinite detention of a 19-year-old man accused of attempting to carry out terrorist attacks targeting Americans. In the ruling delivered Thursday, the judge also said that there is enough probable cause to continue the case against Justin Nojan Sullivan, who was arrested last month and is believed to be an Islamic State group sympathizer, according to media reports.

According to an earlier FBI statement, an undercover agent learned that Sullivan planned to buy a semi-automatic rifle at a June 20 gun show in North Carolina and use it as part of his plot to kill “1,000 Americans.” Sullivan was arrested a day before the show.

“We received information that Mr. Sullivan was engaged in ISIS-related activities,” FBI agent Corey Zachman reportedly said during the preliminary hearing. However, very little evidence was presented in the court on Thursday.

“The techniques we used to get that information are very sensitive,” Zachman reportedly said, adding that a lot of the “classified” information cannot be discussed in an open court.

The FBI first came to know of the alleged plot in April, when his father Richie Sullivan called the authorities to complain that his son was destroying religious items in their home. Later, in early June, when an undercover federal agent reportedly contacted the Morganton teen, he disclosed that he was a Muslim convert and that he was looking for possible targets to attack between June 21 and June 23.

“Our attacks need to be as big as possible,” he reportedly said. “We can do minor assassinations before the big attack for training.”

He also expressed his desire to procure biological weapons and create an “Islamic State of America.”

Justin Nojan, who now faces charges of attempting to provide material support to ISIS, is currently being held at Madison County's jail, according to local media reports. 

In May, FBI Director James Comey warned that "hundreds, maybe thousands" of Americans were receiving recruitment messages from ISIS sympathizers and militants, mostly through Twitter and other social media platforms. 

"It's like the devil sitting on their shoulders, saying 'kill, kill, kill,'" Comey reportedly said.