Peter Petigrew disse a Voldemort, que presumivelmente disse a Lucius. Sirius especula isso em Ordem da Fênix .
“Because the Ministry of Magic’s still after me, and Voldemort will know all about me being an Animagus by now, Wormtail will have told him, so my big disguise is useless. There’s not much I can do for the Order of the Phoenix…or so Dumbledore feels.”
-- Sirius Black speaking to Harry (Order of the Phoenix, Chapter 5)