Can the Unforgivable Curses be Done Non-Verbally?

Question

Are there any examples of the three Unforgivable curses -- Cruciatus, Imperius, and Avada Kedavra -- being done non-verbally?

I checked Half-Blood Prince, chapter 9, The Half-Blood Prince, where Snape holds the sixth years' first Defence class and they begin non-verbal spells, but the Unforgivables are not mentioned.

Are they just too intense to do non-verbally, or would they possibly be able to be cast silently with enough concentration?

Canon examples would be great! Please no HP Wikia answers.

Answer 1

Voldemort very likely did it, when he started dueling Dumbledore in the Ministry Atrium in OotP:

"I have nothing more to say to you, Potter," he said quietly. "You have irked me too often, for too long. AVADA KEDAVRA!" [ Note that he verbalized it here - DVK]
Harry had not even opened his mouth to resist; his mind was blank, his wand lay uselessly on the floor.
...

... (and then, Dumbledore appears and blocks the AK with the statue)...

And then he breathed, “Dumbledore!”
Harry looked behind him, his heart pounding. Dumbledore was standing in front of the golden gates.
Voldemort raised his wand and another jet of green light streaked at Dumbledore, who turned and was gone in a whirling of his cloak.

The "another jet of green light" - and the nature of the situation - pretty unambiguously point to the spell aimed at Dumbledore being Avada Kedavra as well.

However, it is pretty explicit that the Avada Kedavra aimed at Harry was said outloud, and very explicitly NOT stated to have been said aloud when aimed at Dumbledore.

This leads me to conclude that - given JKR's usual written precision - that the second AK was non-verbal.

This makes perfect sense given the details in NominSim's answer -

• If anyone could, Voldemort was it

• Usually (such as aiming at Harry who was explicitly stated to have been unable to resist) there was no need or point to non-verbalize it.

• BUT, when dueling Dumbledore who was an obvious challenge, every last advantage counts, so Voldemort opens with the strongest thing he can - non-verbal Avada Kedavra, to not allow Dumbledore a chance to know what was coming.

Answer 2

When Snape tells everyone to practice by attempting to jinx one another (and repel the jinxes) he doesn't mention any specific spell or any restrictions on spells not being able to be used non verbally. The only thing he mentions is that

Not all wizards can do this, of course; it is a question of concentration and mind power

Hermione also mentions that they are useful in that your opponent won't know what kind of magic you are using:

"Your adversary has no warning about what kind of magic you're about to perform," said Hermione, "which gives you a split-second advantage."

Canon seems to support that there is no restriction on what kind of magic you use. You seem to be only restricted by your own concentration and mind power. However, I could find no evidence of one of the unforgivable curses being done non-verbally, and as they require immense concentration in and of themselves, it is unlikely that any but the most skilled wizards would be able to perform them silently. (Voldemort is a likely candidate for possibly being able to however, he seems to revel in the fear instilled in a victim just by hearing the curse be spoken to them.)

The closest evidence I could find contrary to this belief was during Dumbledore's arrest attempt:

As Dumbledore spoke, Harry heard a rustle behind him and rather thought Kingsley whispered something.

(later)

"Unfortunately, I had to hex Kingsley too, or it would have looked very suspicious," said Dumbledore in a low voice. "He was remarkably quick on the uptake, modifying Miss Edgecombe's memory like that while everyone was looking the other way-..."

This particular situation would have been the perfect time for Kingsley to use a non-verbal spell, as if he were caught modifying her memory it would have been potentially disastrous. However, this is before non-verbality in spells really comes to fruition, and I suspect the artistic merits of Harry hearing a whisper would supersede a non-verbal spell in any case. (I should point out of course that not using a non-verbal spell is in no way evidence that it can't be used).

In conclusion, there isn't a time in canon when an Unforgivable Curse is explicitly stated to have been used non-verbally, but based on the canon evidence of non verbal spells it seems that they could potentially be used that way.

Answer 3

Just watched Fantastic Beasts: The Crimes of Grindelwald. Minor spoilers: lots of nonverbal Avada Kedavra. Note that J.K. Rowling wrote the screenplay for the film, the second of the prequel so far.