Whoops. It seems my memory was playing tricks on me; that’s not actually stated in the story. The scene I was thinking of was:
All prisoners are tested before their arrival at Azkaban; and if they are Animagi, their Animagus form is destroyed. But we had not considered that someone protected by a Patronus Charm while taking the potion and performing the meditation, might be able to become an Animagus after they went to Azkaban—”
“I understood,” Severus said, having by now put on his customary sneer, “that the Animagus meditation required considerable time.”
“Well, Mr. Snape,” Thicknesse barked, “records show that Bellatrix Black was an Animagus before she was sentenced to Azkaban and her form destroyed; so maybe her second meditation didn’t take as much time as her first!”
Reference?
Whoops. It seems my memory was playing tricks on me; that’s not actually stated in the story. The scene I was thinking of was: