June 06, 2016 11:10 AM

brightcove.createExperiences(); Bobby Brown says he and Whitney Houston failed their daughter Bobbi Kristina Brown.

“We could have been better. We should have been better,” Bobby Brown said in a candid interview with ABC News’ Robin Roberts, set to air on 20/20 Tuesday.

Bobbi Kristina died last July at age 22 after being found unresponsive in her bathtub in January 2015. The Fulton County Medical Examiner found she died from a combination of drowning and drug intoxication. A toxicology report found marijuana, alcohol, “a cocaine-related substance,” sedatives, anti-anxiety medications and morphine in her system when she was found.

“It’s not a mystery to me. The same thing that happened to my daughter is what happened to Whitney,” Brown told Roberts.

Despite accepting some blame for himself, added that only one person was around both Houston and Bobbi Kristina near the time of their deaths: Nick Gordon.

Gordon, 26, was Bobbi Kristina’s boyfriend who lived with her at the time she was found unresponsive. He has denied all wrongdoing and has not been charged by authorities in connection with Bobbi Kristina’s death.

“He knows what happened,” Brown says. “I don’t know what happened that night. But he does.”

Houston took Gordon in when he was just 12 years old and going through a difficult time. Though Houston never officially adopted him, Gordon was raised in Houston’s house and was good friends with Bobbi Kristina before they started dating after Houston’s death.

“Regardless of what everybody thinks, I made Krissy and Whitney’s last few years on earth as happy as they would be,” he said in an April episode of The Dr. Phil Show. “I gave them somebody to trust to, to talk to, to be there, someone genuine.”

Following Gordon’s appearance on the talk show, Brown joined his daughter’s conservator as a plaintiff in the $50 million wrongful death civil suit against Gordon.

The renewed motion states that “the defendant ignores this Court’s mandates but freely discusses his role in the incident which led to the horrific injuries to Decedent Bobbi Kristina Brown on public television” and “fully and freely discusses the version of the facts and circumstances which gave rise to this litigation.”

Gordon’s lawyer has filed motions denying any involvement in Bobbi Kristina’s death.

“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 said. “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.”

The 20/20 interview will air at 10 p.m. on Tuesday, June 7, on ABC.

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