"Essentially you have someone driving under the influence and they end up killing four persons. That's the reason for that homicide charge," Gibbs told "Good Morning America" on Monday.
In Oklahoma, second-degree murder charges are warranted when someone conducts an act that's "imminently dangerous to another person" but does so without premeditation. Each count is punishable by at least 10 years in prison.
Chambers' attorney, Tony Coleman, told NBC's "Today" show Monday that Chambers had "no real response whatsoever" when he told her that four people died as a result of the crash. He said he believes she is mentally ill and said she was hospitalized two years ago for an undisclosed mental illness.
Coleman said Chambers' family is "absolutely devastated" by the crash.
"Their thoughts and their prayers seem to be more-so focused on the victims and the family members of the victims of this horrible incident, and that's something that they wanted to make sure was communicated over and over again," Coleman said.
On Sunday, Coleman said there was no indication that Chambers had been drinking before the crash. Police are awaiting blood tests to determine whether Chambers was impaired by drugs or alcohol.
"I absolutely can rule out alcohol," Coleman said.
He said he spoke with Chambers for about an hour.
"During that entire interview, I was not satisfied at all that I was communicating with a competent individual," Coleman said.
He said Chambers was at work before the crash and that she doesn't remember much of what happened, only that she felt extremely confused as she was removed from the car.
"She could have even blacked out," Coleman said.
Chambers' father, Floyd Chambers of Oologah, told The Oklahoman newspaper Saturday that he couldn't believe his daughter was involved and that she was not an alcoholic. He couldn't be reached for comment Sunday by The Associated Press.
Witnesses described seeing people struck by the car go flying through the air and landing on the road. Three adults and a 2-year-old boy were killed and at least 46 other people were hurt, including many children.
Authorities have not released the dead child's identity. The dead adults were identified as Nakita Prabhakar Nakal, a 23-year-old MBA student from India at the University of Central Oklahoma in Edmond, and a married couple, Bonnie Jean Stone and Marvin Lyle Stone, both 65, of Stillwater.
CBS affiliate KOTV reports Stone was an Emeritus professor in the Department of Biosystems & Agricultural Engineering. His wife Bonnie Stone worked for OSU for more than 33 years.
"He was loved by students and one of the best teachers we had," said Ron Elliott, the former head of the Biosystems and Agricultural Engineering Department at OSU. "He just really had a gift for connecting with students and helping them learn," Elliott said in a telephone interview.
Konda Walker, an OSU graduate who was in Stillwater with her sister to celebrate homecoming, said she was only about 50 feet from the crash scene.
She said it took her a few seconds to process what had happened. There were bodies and injured people lying "all over the place," Walker said.
"One woman was a crumpled mess on the road. They turned her over and started CPR. We realized she didn't make it," she said.
Among the injured were nine children 10 years old or younger.
At the corner of the intersection where the suspect's car came to a stop, a makeshift memorial grew Sunday with balloons, flowers, stuffed teddy bears and candles with black and orange ribbons tied around them, for the school's colors. A handmade sign read, "It's always darkest before dawn. Stay strong."
Anthea Lewis had tears in her eyes as she placed a child's hat with an Oklahoma State University logo at the base of the memorial.
"I've lived here my whole life, and this blows my mind," she said.
Hundreds gathered for a vigil at the campus Sunday night.