A devastated father whose estranged wife murdered their two young children in Alabama last week amid a toxic custody battle said he's a broken man.

"I'm dead inside," Derek Johnson told Fox News Digital. "I feel lost. I have nothing, absolutely nothing, and I don’t know how I’m going to recover. I just know I’m going to miss them all."

Derek found his 37-year-old wife Nancy Johnson, Jacob, 2, and Mia, 5, inside the family's home Sept. 28 in Semmes, about 20 miles west of Mobile.

He had returned to the house the day before but was living in an RV on the property in accordance with their separation agreement.

OHIO WOMAN KILLED MOM WITH FRYING PAN, KNIFE OVER BAD GRADES

Nancy Johnson, left, drowned her children Mia, 5, and Jacob, 2, after an Alabama judge ordered her to share custody with her estranged husband, Derek Johnson, police say. (Facebook)

It wasn't until the next morning when he knew something was terribly wrong. When he awoke, he didn't hear the voices of his children giggling and playing.

"I had a really bad feeling," he said. He drilled out the deadbolt of the home's French doors, which crashed to the floor.

"I had so much planned for my kids. My purpose in life was taken away,"

— Grieving father Derek Johnson told Fox News Digital

"I looked to my right and saw my son and daughter, and they were covered in blankets. It looked like they were taking a nap," the 33-year-old father recalled. " I put my hand on my son’s back to wake him up and realized he was completely stiff and had foam coming out of his nose and mouth."

He looked at his daughter who was in the same condition, but the little girl had also had her throat slit.

"I started panicking," he said.

Derek and Nancy Johnson with their children Mia, 5, and Jacob, 2. (GoFundMe)

Moments later, he found his wife in his closet collapsed on the floor. She had hanged herself with the belt of a robe.

"I went into shock. I was yelling. I called the police," Derek said in a trembling voice. Detectives later told him Nancy had drowned their children in a bathtub, which he noticed was filled with red-colored water.

"But my daughter was fighting my wife, and, so, she cut her throat then put her back under the water," Derek said.

His world started to crumble last Christmas, when Nancy blindsided him by asking for a divorce. They met in 2015 at their church the same year that both his parents died.

"She was my angel," he said he had thought.

But he agreed to her request and moved back to Washington to work for Boeing as a maintenance mechanic so he could pay her child support and alimony.

BUILDING SUPERINTENDENT FATALLY STABS WIFE, KIDS IN MURDER-SUICIDE BEFORE JOB CHANGE: POLICE

Alabama police say Nancy Johnson drowned Mia, 5, and Jacob, 2, so she didn't have to share custody with her estranged husband. (Facebook)

Nancy became increasingly paranoid and erratic. She started calling the police on him, even though he was on the other side of the country. She made wild accusations, including that he had poisoned them, according to Derek.

Alarmed by her behavior, he filed for emergency custody, which he was granted, court records show.

That's when Nancy said she wanted to reconcile and asked him to cancel the emergency order.

"I loved Nancy very much but later realized she was manipulating me," he said. As soon as the order was dropped, she took off to Utah with their kids for nearly three months.

On Sept. 17, he briefly saw his children and gave his son a hug for the first time since early June.

Derek was elated after a Sept. 20 court hearing in Alabama established a nesting agreement. Every week, one of them would live in the house with the children and the other in the RV parked on the property.

SLAIN MICROSOFT EXEC JARED BRIDEGAN'S EX-WIFE TO BE EXTRADITED TO FLORIDA

The Semmes, Alabama, home where Nancy Johnson drowned her two children. (Google maps)

They even had a cordial phone conversation Sept. 26 about the logistics of their new arrangement.

In retrospect, he said, he realizes she planned to kill their children the moment the judge ordered them to split custody.

"She didn’t want me to have the kids. The kids were hers, and if she had to share them, she was going to take them with her," he said.

CLICK HERE FOR MORE TRUE CRIME FROM FOX NEWS

He always knew that Nancy struggled with mental health issues but never imagined she was capable of this.

"She lost her grip on reality, and I had been her anchor," he said. Derek moved back into the family home after members of his church helped clean up the crime scene.

"I had so much planned for my kids," he told Fox News Digital. "My purpose in life was taken away."

CLICK HERE TO GET THE FOX NEWS APP

Mobile County Sheriff Paul Burch said at a press conference the killings were a murder-suicide, and Derek is not a suspect.

A GoFundMe to help Derek with funeral costs has raised $5,900.

Rebecca Rosenberg is a veteran journalist and book author with a focus on crime and criminal justice. Email tips to rebecca.rosenberg@fox.com and @ReRosenberg.