The FBI has released more details of how it shut down the Dark Matter online crime forum.

The agency announced last week that it had shut down its investigations into the forum, which was used to share hacking tools, stolen online information and credit card details.

The FBI has now released details of the investigations, including the fact that one of the site's system administrators was in fact an FBI agent working undercover.

"What has worked for us in taking down spy rings and entire mob families over the years - embedding an undercover agent deep within a criminal organisation – worked beautifully in taking down Dark Matter," said FBI cyber division assistant director Shawn Henry.

The unnamed agent signed onto the site using the alias 'Master Splyntr'. It was his job to monitor the site and only allow criminals who would steal from others, as opposed to 'rippers' who stole from other criminals as well.

"It was a group of people who trusted each other," said the undercover agent. "They did a good job of trying to be secure, and they felt secure. There was honour among thieves, so to speak."

The agent spent between one and 15 hours a day onsite, collecting information on trading activities.

In a two-year investigation the FBI claimed 56 arrests by itself and with overseas police forces. Further investigations into forum members are continuing.