The Dai Li In Book 2

In Book 2, the Dai Li were the secret police of Ba Sing Se. They arrested anyone who talked about the war with the Fire Nation. They were the first "silent earthbenders" we saw in the series, preferring to remain stealthy until the right moment to strike. When Azula came to Ba Sing Se, she took control of the Dai Li and organized a coup to overthrow the Earth King. The key to this was the Dai Li.