What killed the Riddles?

[Wanda Sherratt]

No: HPFGUIDX 104864

I've always supposed that the 3 Riddles were killed by a AK curse, but
I was comparing their deaths with that of Cedric, and noticed a small
discrepancy.  They were said to be found with a look of terror on
their faces, as if frightened to death.  Yet when we've seen an AK in
action, on Cedric, it isn't described that way - Harry looks at "his
open grey eyes, blank and expressionless as the windows of a deserted
house, at his half-open mouth, which looked slightly surprised."  Even
poor Mrs. Diggory tries to console herself and her husband with the
thought that he didn't suffer.  I don't know if this is typical, but
from the way it's described the one time we've seen it in action in
GoF, the AK seems almost like a humane way of killing - certainly
compared to a Cruciatus Curse.  Why would it be so much different for
the Riddles?  If they were witches, I could see how they'd be
terrified if they were facing someone who was clearly going to AK
them, but as muggles, they would have had no idea what was about to
happen.  I thought perhaps their "symptoms" were closer to that which
a basilisk could cause, but judging from the size of the one in CoS,
it's not the sort of thing someone could carry around in their pocket
and pull out when needed. Could they have been tortured, like the
Longbottoms?  Would that leave no mark, being done by magic?

Wanda