Word for a dry laugh

I'm looking for a word to fill in this blank:

“They’re delinquents,” The doctor agrees. “But, don’t worry, we’re finding them, Jannie. You will get the justice you deserve.”

I let out a _______. “Don’t even try to pretend you’re doing this for me, my husband, or anyone else but yourself. You’re just trying to save your own skin."

I'm looking for a specific type of laugh, where it's a scoff that turns into a chuckle. For example, if a mother has a very rambunctious child, and he/she does something that surprises and angers her at the same time, she might do this kind of laugh. Is there a word for this? I don't want to say "dry laugh" or "sarcastic laugh" because there is a bit more too it than those. If there isn't a word for this, is there any other phrases I could use?

Consider sardonic.

(adj.) showing that you disapprove of or do not like someone or something : showing disrespect or scorn for someone or something

[Merriam-Webster]

In your usage,

..

I let out a sardonic laugh. “Don’t even try to pretend you’re doing this for me, my husband, or anyone else but yourself. You’re just trying to save your own skin."

Maybe a snicker.

Give a smothered or half-suppressed laugh; snigger

https://www.oxforddictionaries.com/definition/english/snicker#snicker__2

I would use the word "scoff" because it seems to express some kind of disbelief.