A Michigan woman who was with her 9-year-old twin daughters drove a Chevrolet Impala down a boat ramp into a still, murky part of the Kalamazoo River late Monday, the authorities said. All three were found dead.

By Tuesday morning, state divers had pulled the last body — one of the daughters — out of the water.

Now, investigators are trying to understand what was behind what they are calling an “unbelievable tragedy.”

“That’s the question, ‘Why?,’” said Chief Karianne Thomas of the Kalamazoo Department of Public Safety, which is investigating the deaths. “That’s what everyone wants to know. The family wants to know. I don’t know if we ever will.”