GREENVILLE, Aug. 25 -- When Wladimir Klitschko posted on Instagram on August 21, he addressed his daughter before he addressed the public. "To our daughter Kaya," the retired boxing champion wrote, "I will always speak of her mother with respect and make sure she remembers the person she was."

It was the day Hayden Panettiere would have turned 37. She had died nine days earlier.

The Drug Enforcement Administration joined local authorities investigating Panettiere's death on August 20, adding a federal dimension to a case that law enforcement in Greenville County, South Carolina, has not publicly explained. Panettiere, who became one of American television's most recognizable young faces on NBC's "Heroes" before her years as Juliette Bar...