Official rule text
17.4 Captured fighters and infantry do not belong to any player and are returned only when an ability instructs the capturing player to do so. a Captured fighters and infantry cannot be returned as part of a transaction. b Captured fighters and infantry are not returned as the result of a blockade. c When a captured fighter or infantry is returned, it is placed in the supply.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 17.4