By Frank Siebelt

WIESBADEN, Germany (Reuters) - German police will do more to fight crime committed on the "dark net", they said on Wednesday, days after a gunman killed nine people with a weapon bought on that hidden part of the internet.

"We see that the dark net is a growing trading place and therefore we need to prioritize our investigations here," Holger Muench, head of Germany's Federal Police (BKA), told journalists as he presented the latest annual report on cyber crime.

The dark net, which is only accessible via special web browsers, is increasingly used to procure drugs, weapons and counterfeit money, allowing users to trade anonymously and pay with digital currencies such as Bitcoin, the BKA said.

The man who killed nine people at a shopping mall in Munich on Friday was a local 18-year-old obsessed with mass killings who had bought his reactivated 9mm Glock 17 pistol on the dark web, Bavarian officials said.

The BKA said it had taken five market places in the dark net out of circulation last year. Muench said the BKA did not just want to take the sites offline but also catch criminals using them.

Cyber crime cost Germany 40.5 million euros ($44.5 million) last year, the BKA's report said, a rise of 2.8 percent. Most of the more than 45,000 cases involved computer fraud.

Muench said the figures only represented a small part of the true size of cyber crime.

"If we look ahead we see little relief," he said. "Cyber crime is still a growing phenomenon - you could say almost a growing business, even a growing industry."

Police solved 32.8 percent of cyber crime last year, Muench said, adding that many crimes do not get past the exploratory phase and others go unnoticed or are not reported.

(Writing and additional reporting by Caroline Copley; Editing by Robin Pomeroy)