After taking off from Jidda, Saudi Arabia, his C-47 transport crashed “in a mountainous, desolate area” between Asmara and Addis Ababa, killing Mr. Dennett, the pilot, three soldiers and the State Department’s “petroleum attaché” from Cairo. The crash occurred in a region so remote that “native runners” had to bring word of the accident to the nearest town, the paper said.

It was 40 more years before Ms. Dennett rediscovered her father’s obituary while flipping through a scrapbook in her brother’s attic. Something fishy swam through her stomach.

“I suspected there was more to the story,” she said.

Working first with the National Archives in Washington, then with the C.I.A., she managed to obtain hundreds of declassified documents. But most were drab personnel records, she said. One was not: a copy of the accident report with pictures of the crash. The report, she said, declared in no uncertain terms that the accident was precisely that.

Yet she had always heard stories, usually in whispers, that the crash was sabotage. Her father’s best friend had always said it. So did a former spy she said she met one day with the help of the government archivists.

“When I told him who I was he said, ‘Oh, Dan Dennett, what a loss.’ Then he said: ‘Of course, I know about the plane crash. We always thought it was sabotage but couldn’t prove it.’”

There was no evidence in the record to suggest foul play  except in the twisting logic of intelligence work, which only seemed to enhance her suspicions. Then there was the fact, Ms. Dennett said, that the documents she had received all seemed to stop around the time of the crash. “Here I am, hot on the trail,” she said, “and just when I’m getting to the juicy stuff there’s nothing.”

At this point, her struggles with the C.I.A. began. Requests for information were filed and then denied. Six years passed in the back-and-forth. In 2005, with what she said was reluctance, Ms. Dennett sued the C.I.A. to obtain the entirety of her father’s secret file.