Milwaukee ? A 20-year-old grocery-store clerk who authorities say amused himself by posting prank Internet warnings of terrorist attacks against NFL stadiums was arrested Friday on federal charges that could bring five years behind bars.

Jake J. Brahm was accused of writing that radioactive “dirty bombs” would be detonated this weekend at seven football stadiums. He admitted posting the same threat about 40 times on various Web sites between September and Wednesday, authorities said.

The Wauwatosa man surrendered to federal authorities and appeared in court in Milwaukee later in the day.

“These types of hoaxes scare innocent people, cost business resources and waste valuable homeland security resources. We cannot tolerate this Internet version of yelling fire in a crowded theater in the post-9/11 era,” said U.S. Attorney Christopher Christie in Newark, N.J., where Brahm was charged in a sealed complaint filed Thursday. One of the stadiums mentioned was Giants Stadium in East Rutherford, N.J.

Brahm was charged with making a terrorist threat over the Internet, which carries a maximum five-year prison sentence and $250,000 fine. When the potential sentence was read in court Friday afternoon, his mother, Valerie Brahm, with whom he lives, closed her eyes and put her hand over her mouth.

U.S. Magistrate Patricia Gorence released Jake Brahm without bail, citing his spotless record and strong community ties. She prohibited him from using the Internet or traveling outside Wisconsin and New Jersey, where his lawyer said he plans to make a court appearance.

When reporters asked him for comment after the hearing, Brahm smiled and laughed but said nothing.

Brahm’s attorney, Patrick Knight, said after the hearing that his client was shocked by the way the threats were received. He described Brahm as quiet and gentle and said he had not seen anything to indicate Brahm meant any real harm.