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Nick Gordon, Bobbi Kristina Brown, Bobby Brown

WireImage/FilmMagic

Bobby Brown doesn't think Bobbi Kristina Brown's passing is a mystery at all. In fact, he believes that both his daughter's and his ex-wife Whitney Houston's death were the fault of one person and one person only: Nick Gordon.

In an interview with ABC's Robin Roberts, the singer opens up about the loss of his 22-year-old daughter and why he continues to blame her boyfriend—who was once considered Houston's adopted son—for it.

"It's not a mystery to me," Bobby Brown told Roberts. "The same thing that happened to my daughter is what happened to Whitney...There's only one person that was around both occasions. Only one person who says they were there to protect them…and he didn't."

For this reason, Bobby has filed a $10 million wrongful death lawsuit against Gordon. However, the 20-year-old continues denying his involvement and his attorneys have called the lawsuit "slanderous" and "meritless."

Despite putting the fault on Gordon, Bobby does feel he and Houston could have been better parents. "We should have been better," Brown admitted to Roberts. "We could have been better." 

Now he has to continue living with what he describes as one of the most difficult moments of his life: telling Bobbi Kristina to let go.

"The last thing I told her, I just said, 'Let go.' I said, 'It's OK, you can go with your mom, daddy'll be all right. I love you. You can rest now,'" Brown revealed. "Hardest thing I had to do in my life was tell my daughter to let go…I don't wish that on my worst enemy or any enemy. That is the worst pain in the world."

Bobbi Kristina passed away on July 26, 2015, after being found face-down in a water-filled bath tub with drugs and alcohol in her system. She spent six months in a coma before her death.

Bobby's full 20/20 interview will air Tuesday at 10 p.m. on ABC.