"We never forgot Cologne," Elizabeth Reichert told the Kölner Stadt-Anzeiger newspaper by phone from her home in Philadelphia.
She said she and her Jewish husband Arnulf Reichert both grew up in the western German city. They met in 1944, she recalled, when Arnulf lived in hiding to avoid being discovered by the Nazis.
She had known many Jews who had lived secretly during the war in Cologne.The pair married a year after the war ended and briefly moved to Israel before settling in the US, where they lived the American dream and made their fortune.
Reichert said she worked as a hairdresser, while her husband took a job for a wholesaler selling pets and pet supplies, before setting up his own business and making millions.
Shortly before her husband died in 1998, the childless couple agreed to bequeath their money to the Cologne Zoo after their deaths.
"When you start thinking about who you want to leave your money to, memories play a big role. With the zoo, the money is well spent," Reichert said.
The couple had already shown their affection for the zoo in 1954, when they gifted a soft-shell turtle.
The considerably larger donation this time will come into effect after Reichert's death, when a foundation named after her husband will provide the zoo with an annual payment.
The zoo's director, Christopher Landsberg, said he was taken aback when he learned of the windfall from across the pond.
"I nearly fell off my chair," he told the DPA news agency.