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The lawyer representing the woman charged with four counts of second-degree murder after she plowed through a crowd of people at Oklahoma State University's homecoming parade said Sunday he doesn't believe his client was intoxicated — but that she has showed "warning signs" of a mental illness.

Adacia Chambers, 25, was arrested and charged with driving under the influence Saturday after she plowed through revelers on the sidelines of the Stillwater event, killing four and injuring dozens more. The second-degree murder counts were added Sunday, Stillwater police said.

Adacia Chambers in an arrest photo. Stillwater, Oklahoma, Police Department

"She wasn't a drinker. She didn't do drugs," Chambers' attorney, Tony Coleman, said at a news conference Sunday night.

Coleman said he interviewed Chambers for about an hour Saturday night and that even though "her behavior wasn't consistent with drunken stupor ... I was not satisfied that I was communicating with a competent individual."

Coleman said on TODAY on Monday morning that Chambers had “no real response whatsoever” when he told her four people had died, and that’s what solidified his belief that the 25-year-old was “lacking capacity.”

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"In my opinion, Ms. Chambers suffers from a mental illness," Coleman said Sunday. "The person I met with last night did not resemble a monster by any means."

He said Chambers doesn't remember the crash and may have blacked out during or after the incident.

Coleman said Chambers' family and her friends expressed to him that she had shown "warning signs" of mental illness, including uneasiness, uncertainty and trouble sleeping.

"I'm satisfied, based on my experience, that she has the characteristics of someone who has a mental illness. It's just that simple," Coleman said. The attorney said ordering a mental evaluation is "the first thing I'm going to do."

Chambers’ father “did in fact share with me that there were incidents as recently as two years ago where Ms. Chambers was hospitalized and perhaps evaluated,” Coleman elaborated said on TODAY. He said he will present any results of those tests “when necessary.”

Coleman said she may have been on a “cocktail of medications” when she was hospitalized, but she wasn’t on them at the time of the crash.

Chambers is set to be arraigned Monday, Stillwater police said. Results of a toxicology test are pending.

Related: Retired Oklahoma State Professor, Wife Among Dead in Homecoming Parade Crash

Chambers' family is "supportive" and will likely be at the arraignment, Coleman said. "Their thoughts and their prayers are with the family and the victims," Coleman said. "Everyone's absolutely devastated — it's a word that continues to be echoed."

Forty-seven people were injured in the crash, which shocked the crowd at OSU's homecoming parade. Four people — including a 2-year-old boy — were killed.

A candlelight vigil at OSU Sunday night drew more than 1,000 people. Counselors and therapy dogs are on campus for students and staff on Monday, according to the school.