Bobby Brown to discuss Bobbi Kristina and his new book on "20/20" with Robin Roberts

Credit: Jennifer Brett

Credit: Jennifer Brett

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"The hardest thing I've ever had to do in my life was to tell my daughter to let go," he said.

In this preview Brown, still grief stricken, notes that his daughter and his ex-wife died in tragically similar ways.

"There's only one person who was around both occasions," Brown said.

"Nick Gordon is who you're referring to," Roberts responded, noting the wrongful death lawsuit filed by Bobbi Kristina's estate. Gordon's legal team has blasted the lawsuit as slanderous and meritless.

The lawsuit  alleges financial impropriety and severe physical harm .

"He knows what happened," Brown said. "I don't know what happened that night, but he does."

In March, Gordon's legal team issued a statement saying her death was “was an accident… or even a suicide, but not a murder,” and sought an end to the specter of possible legal or criminal proceedings involving him.

No charges have been filed against anyone in connection to Bobbi Kristina's death.

Here's the "20/20" preview:

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