The Authentication Server (AS), which is part of the Key Distribution Center (KDC), is responsible for issuing the Ticket Granting Ticket (TGT) after the initial authentication process. The TGT is used for obtaining service tickets, which in turn are used to access services without needing to re-authenticate with the user's credentials. The Ticket Granting Server (TGS) is responsible for issuing service tickets based on the TGT and is not involved in the initial issuance of the TGT itself. The Domain Controller is a server that responds to security authentication requests but does not directly issue the TGT in a Kerberos system. The Service Principal Name (SPN) is a unique identifier for a service instance on the network but is not a component that issues any tickets.