How were the Death Eaters broken out from Azkaban in Order of the Phoenix?
From the Daily Prophet:
‘We find ourselves, most unfortunately, in the same position we were two and a half years ago when the murderer Sirius Black escaped,’ said Fudge last night. ‘Nor do we think the two breakouts are unrelated. An escape of this magnitude suggests outside help, and we must remember that Black, as the first person ever to break out of Azkaban, would be ideally placed to help others follow in his footsteps.
Order of the Phoenix - page 481 - Bloomsbury - chapter 25, The Beetle at Bay
Samantha: Was Snape the only Death Eater who could produce a full Patronus?
J.K. Rowling: Yes, because a Patronus is used against things that the Death Eaters generally generate, or fight alongside. They would not need Patronuses.
J.K. ROWLING - WEBCHAT 07.30.07 - THE LEAKY CAULDRON
I'm going to guess Voldemort did not break out the nine Death Eaters himself -- it's not his style to do the dirty work. I'll further guess that it's possible a group of Death Eaters who were not incarcerated was sent to break the nine out of Azkaban. Assuming this scenario is basically possible and accurate, how were the Death Eaters broken out of Azkaban, in light of the fact that 1) Azkaban is rumored to be Unplottable¹ and 2) Death Eaters cannot produce Patronuses, and at the time of the break-out the Dementors were still under the control of the Ministry?
I'd like a canon-based answer, such as from the books, J.K. Rowling interviews, or Pottermore.
¹I cannot find verification that Azkaban is Unplottable in any of the J.K. Rowling interviews I have bookmarked or in canon. The HP Wikia says Azkaban is Unplottable, but I would prefer canon verification. I'm sure I've heard that Azkaban is Unplottable before, from another source, but I just cannot find it this morning! On the other hand, if Azkaban is Unplottable, how would any Death Eater know how to get there to facilitate a break in to begin with?