Produced by Alec Sirken, Clare Friedland and Avi Cohen
[This story was originally broadcast on May 20, 2011. It was updated on June 16, 2012.]
(CBS) LIMA, Ohio - As Dr. Mark Wangler mourned the death of his wife, Kathy, he found comfort in his church. He also found comfort in an old friend, Esther Erkman.
"At first, it was just lonely and devastating," he tells "48 Hours Mystery" correspondent Peter Van Sant. "The thing with Esther and I was--we had known each other for a long period of time, not romantically, you know, we had just known each other."
In fact, Esther and her husband had been friends with Mark and Kathy for years.
"Yeah, I saw them many times; they would go out with us, my husband and I on many occasions," says Esther.
Esther, a psychologist with a Ph.D., had great respect for Mark, a top anesthesiologist in Lima, Ohio.
"He's a good doctor. He's a requested doctor. He practiced for, you know, 29, 30 years with, you know, never losing a patient, never being sued for malpractice," she says.
They were brought together by an odd twist of fate. Esther had lost her spouse as well, when he underwent a gender change operation and began living life as a woman.
"Some of my friends tell me I should write a book," Esther says. "But then, I challenge them and ask, 'would it wind up in the autobiography or the fiction section,' because nobody would believe it."
The couple began seeing each other and soon fell in love.
"As people sail through life, you know, in a sense he's the anchor that I need and I'm the wind in his sails," Esther explains.
But Mark was still haunted by what he has always said was a bizarre accident when Kathy was killed by carbon monoxide poisoning Labor Day weekend in 2006.
"It happened to be a Sunday. So Kathy and I got up and went to church together," says Mark.
On that night the two were sleeping in separate bedrooms.
"I would go to bed earlier because I get up earlier to go to work and Kathy was a night owl," he explains.
Mark was sleeping on the first floor in the master bedroom.
"The next thing that I remember is waking up - or being woken up by an alarm," he tells Van Sant. "We had the carbon monoxide alarm down in the basement."
Mark says his natural gas water heater had malfunctioned, sending poisonous carbon monoxide into the duct work of the house.
Kathy was sleeping on the second floor. And horribly, with her door closed, it became a gas chamber.
Mark says he began staggering around, feeling strangely woozy and disoriented - the classic signs of carbon monoxide poisoning.
"I was nauseated. I remember vomiting somewhere along the line as I was going through the house," he recalls. "And I then go up to check on Kathy. ...She was sleeping on one of those inflatable mattresses."
Asked if she was breathing, Mark replies, "I guess she was at that point. ...I couldn't wake her up."
Mark quickly calls 911: My carbon monoxide detector is going off and my wife is having a seizure! ...I'm going to start opening all the windows and stuff, please get here quickly.
"I did do CPR on her," Mark tells Van Sant.
The paramedics rushed in, but just 38 minutes later, Kathy was pronounced dead at the hospital. Their youngest son, Aaron, who was away at college, received a call with the tragic news.
"I just cried the whole time...and it was very like...I can't believe this is real, you know like, you can't believe that your mom's gone or that even one of your parents - and especially I was 21 at the time. You don't expect to lose a parent so young."
Aaron and his older brother, Nathan, rushed to Lima to be with the family.
"Just devastating...we just sobbed on each other's shoulders," Mark says. "We bawled and bawled."
The only reason he survived, Mark says, is blind luck. He says Kathy caused the toilet in the master bathroom to overflow. Mark had opened a window and turned on a fan to air the room out.
Mark was questioned by police, but not charged.
With Esther by his side, he slowly moved on with his life and returned to work.
"You know just having somebody, an anchor to point to sanity in my life, was such a blessing," says Esther.
Fourteen months after Kathy's death, they were married. They settled into a life focused on each other and their church, traveling to Zambia on aid missions.
"We've been providing the means to have water wells drilled in that country, which just has a huge impact," says Mark.
But back in Lima, the newlyweds lived under a cloud of suspicion.
"You've talked about insinuation and you know there's sort of a whisper campaign out there - that suggests you and Mark were having an affair prior to Kathy's death. Is that true?" Van Sant asks Esther.
"No," she replies. "He didn't even hold my hand, Peter, until after my divorce was final. That's the kind of man he is."
While Mark believed the investigation was over, Allen County Prosecutor Jurgen Waldick never closed the case - always troubled by Mark's story.
"On the very night that your wife is killed by carbon monoxide, you happened to be sleeping in a bedroom with the window open and a fan going," says Waldick.
911 Operator: OK, is she still conscious, I mean is she breathing?
Mark Wangler: No, I think she stopped breathing.
And Waldick was bothered by Dr. Wangler's behavior on the 911 call.
"...at the insistence of the 911 operator only does he go back and checks to see whether she has a pulse or whether or not she is breathing," he says.
911 Operator: Sir, I need you to check for me.
Mark Wangler: No, she's not breathing!
911 Operator: Does she have a heartbeat?
Mark Wangler (screaming): No, she doesn't!
"If things looked so fishy, why wasn't Dr. Wangler charged with the crime?" Van Sant asks Waldick.
"We continued the investigation. We had our suspicions," he replies.
"Is there a part of you that wonders, 'Could he have done this?'" Van Sant asks Aaron.
"No. Never. That's never even crossed my mind," he replies. Aaron says he "never once" doubted his father.
Asked if he murdered his wife with carbon monoxide, Mark tells Van Sant, "Absolutely not. No."
Van Sant asks Esther, "Are you living with a killer?"
"No - no," she says. "You know, I - I - if I was living with the evil genius, I think I might be one of the first to know," she says.
"In fact, you have a Ph.D. in psychology?"
"Yes. I do."
"And as part of that, you can read people pretty well I take it?"
"I'd like to think so."
Kathy's family is convinced Mark has everybody fooled. That the man trained to put people to sleep had come up with a textbook murder.