Additional Director General of Police (Headquarters) Sunil Kumar said the letter was posted from Giridih in Jharkhand and received at the Assembly yesterday, following which a case was registered.
He said a police team was constituted under Deputy Superintendent of Police of Secretariat to investigate the matter.
Later, Senior Superintendent of Police of Patna said the police team went to Giridih to investigate and found that the letter was fake, Kumar added.