I'll just cut-and-paste directly from Wikipedia's entry on Cass:
"During this time, Cassandra also went undercover for Batman in the Justice League Elite, working under Sister Superior to track and eliminate metahuman threats to the population. She worked with Batman's old fellow JLA members Green Arrow and Flash, and formed a certain bond with Coldcast, who was the first person she revealed her real identity to. Although he was subsequently accused of murder, she and the rest of the team soon realized that he had been manipulated by renegade Elite member Menagerie, who was also being manipulated by the spirit of Manchester Black as he tried to drive his sister to destroy London. As the JLA fell, the Elite, united by the spirit of the deceased Manitou Raven, freed Vera and vanquished Black, although the team disbanded after this last mission."