The son of a Boston police captain arrested this month and accused of planning to bomb a college cafeteria on behalf of the Islamic State militant group pleaded not guilty on Wednesday to criminal charges.

Alexander Ciccolo, 23, did not speak during his federal court appearance other than to enter his plea. He was arrested on July 4 after illegally receiving four guns from a person working with the U.S. Justice Department, which had placed him under surveillance after his father alerted authorities to his activities.

Ciccolo told an informant that he wanted to build bombs similar to those detonated at the Boston Marathon in 2013 and federal agents saw him buy a pressure cooker similar to the ones used in that attack, which killed three people and injured more than 260, according to federal prosecutors.

Authorities also said they found partially built bombs at Ciccolo’s apartment that were filled with a mix of shredded Styrofoam and motor oil, a combination that federal prosecutors said Ciccolo believed would stick to victims’ skin, causing severe injuries.

Ciccolo has been held in solitary confinement, his lawyer, David Hoose, said after the hearing.

“He’s holding up as well as you can expect a young man who’s never been in custody before, now finding himself locked up 23 hours a day,” Hoose told reporters.

Ciccolo pleaded not guilty to two criminal counts in U.S. District Court in Springfield, Massachusetts – illegally receiving four guns, including two military-style assault rifles, and stabbing a nurse with a pen while he was being booked at a jail in Western Massachusetts after his arrest.

He was prohibited from owning firearms due to a prior criminal conviction.

Federal prosecutors have not charged Ciccolo in the planned attack and Hoose said he did not know if additional charges would follow.

“I can’t comment on whether he is connected to terrorism,” Hoose said. “I have not been given a lot of information.”

Ciccolo’s mother and stepfather attended the arraignment, but left the courthouse without speaking to reporters.

A federal judge unsealed the charges against Ciccolo on July 13 and ordered him held without bail the following day.

Ciccolo could be sentenced to up to 20 years in prison if convicted. In a video taken after his arrest and played in court this month, Ciccolo defended “Islamic State’s” hostage executions and agreed with an FBI interviewer’s assertion that in his view all Americans should be considered “enemies.”

U.S. authorities have expressed increasing concern about “lone wolf” attacks by citizens who become adherents of ISIS and other militant groups, many of which have robust online propaganda operations.