A police officer working for Europol exposed sensitive data about security investigations to the internet.
The European Union's law enforcement agency acknowledged the error ahead of a Dutch documentary's broadcast.
The TV programme Zembla said it had found more than 700 pages of confidential dossiers, including details of terrorism probes, on a hard drive linked to the net.
It said the networked drive was not password protected.
Europol said it had launched an investigation into the matter.
"Although this case relates to Europol sensitive information dating from around 10 years ago, Europol immediately informed the concerned member states," a spokesman for the law agency said.