His brother Karar Mustafa Qasim, 19, who had moved to Norway as an asylum-seeker, was among the victims of Breivik's shooting rampage at a youth camp, Nordtun said.
"I took off my shoe, got up, shouted at the killer, got eye contact with him and threw the shoe," Qasim was quoted as saying by the Norwegian newspaper Aftenposten.
"He was alone in Norway, without family," Qasim said of his brother. "The killer took his life. And he ruined the life for me and the family. I have traveled from Iraq to Norway to be in court. And it has made an enormous impression on me."
Throwing of shoes to insult someone has long been a form of protest in many countries, but the practice gained widespread attention when an Iraqi threw his shoes at then-President George W. Bush at a televised news conference in Baghdad in 2008 during the Iraq war.
Witnesses said forensic experts were going through autopsy reports for some of the victims when a man in the second row suddenly stood up and threw a shoe at a desk where Breivik and his defense lawyers were seated.
"He shouted, 'You killer, go to hell.' And repeated it several times" in English, said Mikaela Akerman, a Swedish journalist who was in the courtroom.
The shoe hit one of Breivik's defense lawyers, but she was not hurt.
Breivik remained calm and "smiled a little" as he watched security guards apprehend the man and take him out of the courtroom, Akerman told the Associated Press.
"He keeps shouting and is crying heavily as he's being led out," Akerman said. "Some of the spectators clapped their hands. Some yelled 'Bravo.' Many others started crying."
Qasim's attorney said the outburst was unplanned.