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, Victoria 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.

"When you're a 20-year-old, and you roll a snowball, and the next thing you know, it's as big as a house rolling down a hill, it surprises you a bit," Knight said.

Richard Ruminski, the FBI agent in charge of the Milwaukee office, said Brahm thought posting the threats would be funny.

"As I understand it, Mr. Brahm had put out this threat thinking it was so preposterous that no one would take it seriously," Ruminski said. "Unfortunately, he was wrong."

The warnings briefly set off a scare this week, before federal authorities announced the warnings were a hoax.

Wauwatosa police Chief Barry Weber said Brahm had bragged to friends about his actions.

An FBI official in Washington, who spoke on condition of anonymity because the case is still under investigation, told The Associated Press that Brahm acknowledged doing it as part of a "writing duel" with a man from Texas to see who could post the scariest threat.

One of the threats, dated Oct. 12, appeared on the Web site "The Friend Society," which links to various online forums and off-color cartoons. Its author, identified in the message as "javness," said that trucks would deliver radiological bombs Sunday to stadiums in Miami, Atlanta, Seattle, Houston, Cleveland, Oakland, Calif., and the New York City area, and that Usama bin Laden would claim responsibility.

On a separate blog under Brahm's name, the writer posted a photo of a football stadium next to a list of goals for 2006, which included committing a felony and "trying to leave the house at least twice a week."

Brahm worked at Outpost Natural Foods, a co-op near his house. "He was a normal guy. That's all we're going to say at this time," said Jeremy Layman, assistant manager.

Mallory McKenney, who graduated a year before Brahm from Wauwatosa East High School, said: "He was quiet but always seemed friendly whenever we were in a group of people together."

Erik Vasys, an FBI spokesman in San Antonio, said there would be no charges against the Texas man because he did not take part in the writing of the threats.