Update -
Link from reuters.
Heiner Wilhelm from Lower Saxony police said the suspect was not aware of the extent of the damage he had caused. "He had become worried about the impact but he hadn't reckoned on our turning up," he told a news conference in Hanover.
When police went to the man's house they found a home-built computer which contained the source code used to run Sasser.
State prosecutors said the man could be tried in a youth court because he turned 18 only on April 29 and was possibly a minor when he committed the crime of "computer sabotage."
The crime carries a maximum sentence of five years' imprisonment if tried in adult court, but state prosecutor Helmut Trentmann said he could not specify the maximum penalty the suspect could get. Police said it appeared the man acted alone but communicated with other programers. He told officers he wanted to create an anti-virus to other viruses like Mydoom and Bagle.
The man said he created the worm Netsky to fight the viruses but encouraged by all his friends he developed it further and Sasser was created as a modification of Netsky, police said.