In-Q-Tel, the investment arm of the CIA, has invested in Visible Technologies, "a leading provider of social media analysis and engagement solutions". The move is believed to be part of the CIA’s aim to utilise the "open source" information available on social networking sites.

In-Q-tel was set up in 1999 to identify "innovative technology solutions to support the mission of the CIA and the broader U.S. Intelligence Community".

Troy Pearsall, executive vice president of architecture and engineering at In-Q-Tel, said: "Visible Technologies' platform is key to understanding the breadth and depth of the online social landscape.

"Its platform delivers a clear and comprehensive view of complex information, integrating real-time data into a navigable and easy-to-use application that understands the context and tone of online dialogue."

Visible listens in to around half a million sites a day covering forums and blogs. It also crawls commercial sites with forums such as Amazon. The only sites it cannot reach are closed networks such as Facebook.

Customers of Visible receive real-time feeds on a required subject, based on a series of keywords. These are scored as neutral, positive or negative depending on the message and the influence of the writer.

Neither Visible, which expects revenues of about £20m next year, or In-Q-Tel would divulge the size of the investment. According to sources at the company, the money will be used to boost its language capabilities.

Steve Aftergood of the Federation of American Scientists said it was legal to monitor social networking sites but there were restrictions as to how this could be used.

In a Wired magazine report, he said: "Even if information is openly gathered by intelligence agencies it would still be problematic if it were used for unauthorized domestic investigations or operations.

"Intelligence agencies or employees might be tempted to use the tools at their disposal to compile information on political figures, critics, journalists or others, and to exploit such information for political advantage.

"That is not permissible even if all of the information in question is technically 'open source'."

Visible Technologies currently works with clients including Microsoft, Hormel and Xerox.