The RNC's headquarters in the capital was temporarily shut down due to a package said to contain vials of blood.
After the building was evacuated, a rapid security sweep was carried out.
After a while, the RNC headquarters and employees were allowed to return to the building after the police announced that there was "no problem".
No official statement was made about the package delivered by mail.
The yellow tape that the police had drawn on the street where the headquarters building was located during the investigation was removed and traffic was allowed to flow again.