"He's a big mama's boy, you know. His best friend is his kitty cat," John Cornell told CNN's "Anderson Cooper 360."
"There's no way he could have carried out any kind of terrorist plot," the father said. "I think Chris was coerced into a lot of this."
Christopher Lee Cornell, 20, came to the FBI's attention several months ago for alarming social media posts in which he talked about his support for violent jihad, according to a criminal complaint. He allegedly wrote under the alias Raheel Mahrus Ubaydah.
The FBI engaged in an undercover operation, with the help of a person who began cooperating in exchange for favorable treatment on his criminal exposure on an unrelated case.
Cornell allegedly told that source he had been in contact with persons overseas, and that he had aligned himself with ISIS.
He did not think he would receive "specific authorization to conduct a terrorist attack in the United States, but stated that he wanted to go forward with violent jihad and opined that this would be their way of supporting ISIL," the complaint said, using another name for ISIS.
According to the complaint, Cornell wrote: "I believe that we should just wage jihad under our own orders and plan attacks and everything ... I believe we should meet up and make our own group in alliance with the Islamic State here and plan operations ourselves."
He was plotting, with the undercover FBI operative, to set off pipe bombs in the U.S. Capitol and shoot people as they fled, the complaint said.
Cornell and the operative allegedly met in Cincinnati in October and again in November.
He researched the targeted government buildings and the construction of pipe bombs, saved money for the attack and bought weapons and ammunition, the complaint said. Cornell was arrested Wednesday.
His father said his son, who lived at home in Green Township, Ohio, recently took up an interest in Islam but had never mentioned ISIS.
"He explained to me, you know, that Islam wasn't a terrorist group. It was a way of life," John Cornell told CNN's "Erin Burnett OutFront."
"I don't think Chris could hurt a fly," he said. "He's such a loveable, kind person."
According to a law enforcement source, the plan posed no threat to lawmakers. But the complaint indicated that the plot was in its late stages.
The Islamic State in Iraq and Syria, also known as the Islamic State and ISIS, is seeking to create a fundamentalist religious state across Sunni areas in those two countries.