You can use the following FQL to retrieve posts related to a particular event:
SELECT post_id, message FROM stream WHERE source_id = {Event_Id} LIMIT 500
Similarly, you can perform the following multi-query to retrieve the comments:
SELECT text FROM comment WHERE post_id IN
(SELECT post_id FROM stream WHERE source_id = {Event_Id} LIMIT 500)
Replace the {Event_Id} with the Id of your Event.