"He was the one that was administering," she told ABC's Robin Roberts. "I think he is responsible."
Michael Jackson's personal physician, Dr. Conrad Murray, remains the focus of a manslaughter investigation into the pop star's death. Murray has admitted to administering the anesthetic propofol, but has denied giving Michael anything that should have killed him.
Jackson, who has walled herself in silence, fiercely guarding her private thoughts about the death of her beloved brother, Michael, opened up to Roberts in an exclusive interview that will air, Wednesday, Nov. 18 at 10 p.m. ET.
"It's been a tough year," she said. "You have your days where it's just really -- it's hard to believe. And a day doesn't go by that I don't think about him."
Jackson recounted the details of the morning of June 25, before she learned that Michael had collapsed, and her world turned upside down.
"I was at my house in New York. You know, another day. Another morning. And I get a call ... [my assistant] said, 'Your brother's been taken to the hospital. It's on CNN right now,'" she told Roberts. "I called everyone's. There's a line busy or -- someone wasn't picking up. I spoke to mother. I spoke to Tito. I spoke to my nephew Austin. I spoke to my sister La Toya."
Watch Robin Robert's exclusive interview with Janet Jackson, "In the Spotlight," Wednesday, Nov. 18 at 10 p.m. ET.
"I told them to call me when they got to the hospital. And I remember thinking nobody's calling me back, so I tried calling again, and that's how I found out that he was no longer ... I couldn't believe it," she said.
Jackson said the she and the entire family were in a state of disbelief.
"It just didn't ring true to me. It felt like a dream," she said. "It's still so difficult for me to believe. It's, you know, you have to accept what is. But it's hard. You have to move on with your life. You have to accept what is and I understand that."
"My brother's favorite song is 'Smile.' And I thought Jermaine sang it beautifully, beautifully. And that's his favorite song as well. ...There being some sort of a closure, I suppose, at that time," she said, getting emotional.
Janet, who has sold 100 million records and became a five-time Grammy award-winning artist, was the youngest of nine children in the brood. Growing up, she said that she was always closest to Michael.
"We were incredibly close," she said, "A lot of similarities, his love for children and me being a baby. …We would practically do everything together from morning to night every day. Everyday."
She recalled how the two would play after school, feeding the animals together at the family's Hayvenhurst compound in Encino, Calif.
"We'd feed all the animals, took care of the babies. All the animals -- giraffes, mouflon sheep, deer, they had fawned. All kinds of animals, all kinds of birds. And I remember I would come home from school with the hay like I'm going to a ranch," she said.
In later years, she famously wore the key to the animal cages in all her music videos -- a memento from her youth.