How to get categories and tags?

I am writing handler for save_post action, and I need to get list of categories and tags of saved post. Unfortunately this code doesn’t work for me:

function save_post($post_ID)
{
	echo "<pre>\n";
	$cats = wp_get_post_categories($post_ID);
	print_r($cats);
	$tags = wp_get_post_tags($post_ID);
	print_r($tags);
	die("");
}

Both wp_get_post_categories() and wp_get_post_tags() returns empty arrays. Why this code doesn’t work? Is any other way to get categories and tags in save_post handler (or at least count of them)?