The trial of Sven Jaschan, 19, was being held behind closed doors in the northwestern town of Verden because he was a minor at the time of the offense. He entered the courthouse through a side door and did not speak to reporters.

After proceedings began, Jaschan "admitted to the alleged offenses in every detail," court spokeswoman Katharina Kruetzfeld said.

Because defendants do not enter formal pleas under German law, proceedings continue despite Jaschan's confession. The trial was to last three days, with a possible verdict on Thursday.

Authorities said Jaschan already confessed to creating the worm at the time of his arrest in May 2004, about a week after the worm hit public hospitals in Hong Kong, a third of Taiwan's post office branches and check-in desks at British Airways.

The charges, which also include disrupting public services and illegally altering data, carry a maximum sentence of five years in prison, though Kruetzfeld said Jaschan, as a minor, faces a lesser penalty.

Jaschan was arrested at his family's home after Microsoft Corp. received a tip from an informant seeking a reward.
Exploiting a flaw in the company's Windows 2000 and Windows XP operating systems, the worm had raced around the world and caused some computers to continually crash and reboot.

Unlike most outbreaks, Sasser did not require users to activate it by clicking on an e-mail attachment. Sasser is known as a network worm because it can automatically scan the Internet for computers with the security flaw and send a copy of itself there.

Authorities who questioned Jaschan said they got the impression his motive was to gain fame as a programmer. He was arrested sitting at his computer at the house of his mother, who runs a computer store in the small northern town of Waffensen.

The teenager has told officials his original intention was to create a virus, "Netsky A," that would combat the "Mydoom" and "Bagle" viruses, removing them from infected computers. That led him to develop the Netsky virus further �?and to modify it to create Sasser.

Investigators say he had launched a new version of Sasser that was meant to limit the damage just before his arrest.
In their indictment, prosecutors chose the cases of three German city governments and a public broadcaster whose systems were disrupted.

Five suspected accomplices �?including the informant �?also are under investigation, but Jaschan "is the big fish," prosecutor Helmut Trentmann said before the trial.