Are there any studies linking cigarette smoking to Alzheimer's or other dementias?

My FIL smoked for 65 years, didn't get dementia and lived to 87. My dad did not smoke, got dementia and died at 68. Go figure.

I've read over the years of different ideas that MIGHT contribute to a person getting Alzheimer's. Smoking was just ONE of a list that included, not enough fish in the diet, too much tin in a persons body caused from eating food for decades out of tin cans, the inability to process glucose which feeds the brain, and of course heredity. What I personally think is, no one knows for sure what causes it... YET. Both my in-laws smoked for 50 years or so, my father-in-law died 5 years ago but not from smoking since he'd stopped about 15 years before, and he didn't have Alz. My mother-in-law HAS Alz. but she stopped smoking over a decade BEFORE she had Alz. So who knows.