The Helsinki Times article[1 english] says they suspect it was an ex employee.

"Information obtained by Helsingin Sanomat from two different sources indicates that Nokia believes the blackmailer to be a Finnish citizen who participated in the development of the user interface. The suspect was able to obtain the highly-classified encryption key due to a data security vulnerability."

[1] http://www.helsinkitimes.fi/finland/finland-news/domestic/10...