 Aside from mechanisms for the return of fugitives, states have also developed mechanisms for requesting and obtaining evidence for criminal investigations and prosecutions.When evidence or other forms of legal assistance, such as witness statements or the service of documents, are needed from a foreign sovereign, states may attempt to cooperate informally through their respective police agencies or, alternatively, resort to what is typically referred to as requests for "mutual legal assistance" The practice of mutual legal assistance developed from the comity-based system of letters rogatory, though it is now far more common for states to make mutual legal assistance requests directly to the designated "Central Authorities" within each state.In contemporary practice, such requests may still be made on the basis of reciprocity but may also be made pursuant to bilateral and multilateral treaties that obligate countries to provide assistance.