"I do believe the emotional issues came into play quite often in her lifetime," Johnny Huckaby told "Good Morning America's" Chris Cuomo in an exclusive interview.
"She did suffer from depression. She did have issues with .... her self-persona, who she saw herself as," Huckaby, 26, told "GMA."
Watch the full interview on "Good Morning America" Friday, April 17, from 7 a.m. to 9 a.m. EST.
The couple was married in August 2003, when Melissa was five months pregnant. They separated in August 2004 and divorced in September 2005. Melissa was given full custody of their daughter, now 5-years-old.
A volunteer Sunday school teacher, /*Melissa Huckaby*/ was charged Tuesday with murder with the special circumstances of rape with a foreign object, lewd or lascivious conduct with a child under 14 and murder in the course of a kidnapping. Prosecutors say Huckaby raped and murdered /*Sandra Cantu*/, a neighbor in her Tracy, Calif., trailer park, and then stuffed the girl's body in a suitcase and dropped it in an irrigation pond.
"My initial reaction was shock and complete disbelief," Johnny Huckaby said. "It's not something you want to think anyone possibly [is capable] of doing -- let alone somebody that you knew. And that's a mother of your child."
Despite her struggle with depression, Huckaby described his ex-wife as a generally "care-free person" not prone to violence.
Melissa Huckaby, if convicted, could face the death penalty or life in prison without the possibility of parole. Prosecutors have said they have not decided whether they will ask for the death penalty.
Johnny Huckaby, who said he had not talked to his ex-wife for more than three years, is seeking custody of their daughter. He is remarried and has two other children.
He said that when they separated, Melissa Huckaby moved in with her parents in Southern California, so he rarely saw his daughter.
During a custody battle several years ago, Melissa Huckaby accused Johnny Huckaby of alcohol abuse and domestic violence, allegations that he denies.
In the Courtroom
During her April 14 arraignment, Melissa Huckaby reacted most strongly when the rape allegation was read, closing her eyes and appearing to choke back a sob. After the brief hearing, Huckaby was taken back to the San Joaquin County Jail, where she remains on suicide watch.
She was scheduled to return to court April 24, when she is expected to enter a formal plea.
Huckaby's attorney requested a gag order in the case, which will be considered at the April 24 hearing.
After the hearing, Huckaby and Cantu's families were led out of the courtroom separately. When Huckaby's father, Brian Lawless, was asked if he thought his daughter was innocent, he replied, "yes" and said he was "shocked" at the charges.
Sandra's family left shortly afterward and declined to comment.
Sandra disappeared March 27, and after a massive search, her body was found April 6 stuffed inside a suitcase floating in an irrigation pond a few miles from her home.
So far, police have not said how or why they think Sandra was killed.
Melissa Huckaby had been in trouble with the law before the murder charge. When, after six hours of questioning, she was arrested Friday night on kidnapping and murder charges, she was on probation for theft.
Police said that when Sandra was captured on a surveillance camera skipping down the street, she was on her way to play at Huckaby's home.
Police believe the girl was killed shortly after she was seen on the surveillance camera.
Police apparently began to narrow in on Huckaby early last week when investigators from Tracy, Calif., and the FBI began searching the Clover Road Baptist Church. Huckaby had made comments to the local media about her suitcase having been stolen and, later, that she had found a note after Sandra disappeared, apparently from the killer.
Huckaby's Arrest a Surprise
The grief and fears of this small, tight Northern California town of about 78,000 people, which sits about 60 miles east of San Francisco, are now laced with anger at the neighbor the Tracy residents thought they knew, who now stands accused of taking young Sandra's life.
"I hope she rots for this," Amber Austin, a Tracy resident, said last week. "You have no right to take an innocent child's life."
John Hughes Jr., Huckaby's uncle who lives in Whittier, Calif., told The Associated Press that his niece was "from a good home but had hit a rough patch in her life and had moved in with her grandparents in Tracy."
He described Huckaby, who grew up in California's Orange County, as the oldest of nine grandchildren, and said she often played "mother hen" to the younger children at family get-togethers.
He added that because she was divorced with a young daughter, she had difficulty finding and keeping a job.
"She's had her struggles," Hughes told the AP, but there's no way [her grandparents] or anybody would be fearful that anything this horrifying could possible come from that."
ABC News' Laura Marquez, Scott Michels, Sarah Netter, Katy Leaver in San Francisco, ABC News Radio and The Associated Press contributed to this report.