The Committee

You may be looking for Central Committee of Mondas.

The Committee was a group of aliens from the planet Erebus. They took over planets in Signus A, Omega Centauri, and Andromeda. Jack Harkness met them on Kepri 5 in the constellation of Fornax, where he told them that Earth had no intelligent life. Later, the Committee came to Earth and realised Jack had lied to them. One of them assumed the name "Kate" and became the adopted daughter of journalist George Wilson. She told him real things about the Committee to put in his conspiracy theory books. When Sam Hallett got too close to the truth, she killed him and made it look like a suicide. Jack discovered Kate's nature, and went to confront her about Sam's death, but Kate killed George and left Jack to be discovered by the police. Jack fled, and left a recording for the Torchwood team to explain the situation. (AUDIO: The Conspiracy)