The computer that a 20-year-old Wisconsin man used to make an online posting that threatened to detonate radioactive 'dirty bombs' may also have led to his discovery and arrest.

Computer forensics played a key role in the investigation that culminated in the Oct. 20 arrest of Jake J. Brahm of Wauwatosa, Wis. Brahm, who is a grocery store worker, was charged with one count of willfully conveying false information or hoaxes threatening buildings through the use of weapons of mass destruction and radiological dispersal devices. His threat involved setting off bombs in football stadiums in seven states on Oct. 22.

Brahm faces a maximum of five years in federal prison and a $250,000 fine.

"I can't comment on matters still under investigation but the computer forensics side was very important," said Assistant U.S. Attorney Michael Hammer, chief of the Terrorism Unit in Newark, N.J. "You need to make sure that when you're trying to find out who posted [the threat] that you do have a match with the computer that was used. Computer forensics were critical."

Brahm was allowed to voluntarily surrender in Milwaukee, but he's expected to be brought to Newark within the next two weeks to be indicted. One of the threats was against Giants Stadium in East Rutherford, N.J., where the New York Jets were scheduled to play on Oct. 22. The other threatened stadiums were located in Miami, Atlanta, Houston, Cleveland, Seattle, and Oakland, Calif.

Hammer says Brahm posted the threat at www.4chan.org, which runs a public forum. Brahm allegedly admitted to federal authorities that he posted the threat about 40 times on the same site. The threat was then posted on other sites, but Hammer says they're still investigating whether Brahm or someone else did the reposting.

According to the criminal charge, part of the message read, "The death toll will approach 100,000 from the initial blasts and countless other fatalities will later occur as result from radioactive fallout. ... Global economies will screech to a halt. General chaos will rule."

At this point in the investigation, Hammer could not comment on what Brahm might have done to cover the digital trail that would lead from his computer to the Web site where he allegedly made the postings.

''These types of hoaxes scare people and cost businesses resources, and they cause us to have to spend, or waste, valuable law enforcement and Homeland Security resources," says Hammer. "It's like yelling, 'Fire!' in a crowded theater in the Internet age. When people first see it, they don't know what to think. A nightmare scenario for us was if we didn't mitigate the threat and show it to be a hoax, and then somebody lights off a firecracker at one of the stadiums, like Giants Stadium during the Jets game. People are hinky because of this threat and they panic and somebody gets hurt in a stampede. We had several concerns and that was certainly one."

Federal authorities took the threat seriously, says Hammer, though he would not say how quickly they determined it to be a hoax.