EDINBURG – In an echoing hallway inside the Hidalgo County Courthouse, Martin Knell Jr. finally opened up about the murder of his late father, Martin Knell.

“I’m sitting here, over the past three years feeling I should have known. I should have known something was way off track. Of course, I can't go back and undo it. Anyway, it is what it is,” he said.

Knell Jr., who goes by Mark Knell, told CHANNEL 5 NEWS on the phone about two years ago that he wouldn't talk to media on the record until Monica Melissa Patterson was convicted.

On Thursday, Patterson was sentenced to life in prison without parole.

The events leading up to this conviction started at Martin and Thelma Knell's McAllen home in September of 2014.

Knell Jr. was suddenly tasked with finding his sick mother, Thelma, a place to stay. That was after doctors and Adult Protective Services deemed his father unable to care for her.

Knell Jr. lives four hours away in New Braunfels. So when he was told his mother was already pre-approved to stay at the Comfort House in McAllen, he agreed EMS crews should take her there.

“They said Comfort House and I thought well, I can put her in there for a week and then I can get her moved out. Well, I got roadblocked everywhere I turned. Every step I took,” he said. “It was Melissa.”

Knell Jr. and his mother had agreed if she were ever to need assisted living, she would go to Waterford Gardens in Mission.

Patterson told Knell Jr. she would arrange the transfer. The next day, she told Knell Jr. that Waterford Gardens refused to admit his mother.

Thelma Knell died not a month later on Oct. 23, on her 74th wedding anniversary.

Knell Jr. said after his mother’s death, Waterford Gardens told him Patterson lied about their willingness to accept his mother.

“We started getting real serious inclinations about all the lies that we'd been told. They were serious lies,” he said.

Knell Jr. said after his mother's death, he and his father discussed the senior Knell's will, finances and medical situation.

“We talked about him getting to be 100 and what I could do to help him,” Knell Jr. explained.

The father and son spoke on the phone several times a day until Thanksgiving time of 2014. That's when the senior Knell told his son he wanted to spend Thanksgiving with Patterson instead of his family.

Then, his father stopped calling.

“December 3rd, I called him to say hi. My first words to him were 'Hey Dad, how’re you doing?' And he just came back at me with such venom,” Knell Jr. recalled. “Anyway I asked him 'Who's with you?' and he told me Melissa.”

Knell Jr. said after that phone call, his dad stopped answering the phone.

“I never talked to him again,” he said.

Knell Jr. said his father's good friends and neighbors were cut off from communication as well.

During that time, Patterson was visiting Knell often. She took him to the bank to withdraw money and to change his will.

Knell Sr. died Jan. 28, 2015. As a World War II veteran, Knell Sr. was given a funeral complete with military honors.

One month later, Texas Rangers investigators called Knell Jr. and told him a witness had come forward saying Knell Sr. was murdered.

“I never imagined that she would have the ability to murder him. No. Never,” Knell Jr. said.

He questions whether his mother's death was natural.

“I was asked by law enforcement to give them an answer on exhuming on my mom. I told them no,” he said. “I decided the only thing I really could do for my mom and dad was to get justice.”

Patterson's life sentence is the justice he was waiting for.

“I’d been asked about the death penalty early on, whether I wanted to pursue it and I told them no.”

Knell Jr. is prepared to relive this story again in December for Patterson’s co-defendant Angel Mario Garza’s capital murder trial.

He'll also relive it so others can learn from it. He has the following advice for those with elderly parents: know their business and know who else knows it.

“It’s just like we tell our kids 'Watch who you run around with.' Well it’s the same with your parents. You have to turn the table now. You're not the kid anymore, you're the parent,” he said. “So you have to keep an eye on them and you have to know who they're running with and what questions and proddings they're getting from the people they run around with.”

The past three years have been hard on the entire Knell family, he said. Now, the healing has started and with Patterson’s conviction, comes a new peace.

“I forgave her and Mario a long time ago. I had to get that out of me,” he said. “I had to. I couldn’t live with it.”

Link: 2015 Murder Case Youtube Playlist