For two years, Anne Frank's family hid in secret rooms in Amsterdam, knowing that a curtain left open by mistake, a wayward noise or a nervous conspirator's phone call to the Nazis could land them all in concentration camps.

The worst happened on a summer day in 1944, when investigators discovered their secret world behind a movable bookcase and rounded them up.

Of the eight Jewish people seized, seven died before the Holocaust was over, including Anne, whose diary was a testament to the horrors of the Nazi regime. She died of typhus at age 15 at Bergen-Belsen camp in Germany.

For decades, Anne's father, Otto, tried to figure out who tipped off the Nazis — a question historians have debated for 72 years.

Now, the Anne Frank House museum in Amsterdam has put forth a new theory: It was a coincidence...