A Michigan man is behind bars on a murder charge for allegedly killing his wife by spiking her cereal with heroin.

Jason Harris, 44, of Davison, was arrested Tuesday and charged in connection with the death of his wife five years ago.

When Christina Ann-Thompson Harris, 36, died in September 2014 after being found unresponsive in her home, it was ruled an accidental overdose, the Genesee County prosecutor, David Leyton, said at a news conference.

On the day she was found, a neighbor told police that Harris had called asking her to check on his wife.

When the neighbor discovered Christina Harris in her bed and was unable to wake her, she called another neighbor, who is a nurse. The nurse realized the woman had died and called 911.

Christina Ann-Thompson Harris and Jason Harris. via Facebook

By the time police arrived, Jason Harris was there. He told officers that his wife had been battling a cold and when he left for work that morning she had been coughing and in a restless sleep, Leyton said at the news conference Tuesday.

A medical examiner said her death was due to accidental heroin toxicity, a ruling that shocked her family and friends.

Many people who knew Christina Harris, a mother of two children including an infant, said "that she would never use drugs, let alone heroin," Leyton said. "By all accounts, Christina Harris was an incredibly loving mother."

Prosecutors now believe Jason Harris "murdered his wife," the prosecutor said. "We believe that he put heroin into her cereal and milk the night that she died."

Soon after her death, police started to hear new disturbing details about Jason Harris' relationship with his wife. In October 2014, two of his siblings went to police to say that their brother might have been having an affair prior to his wife's death and that they had heard him discuss wanting to get "rid of Christina," Leyton said.

Christina Harris' mother told police that she had been with her daughter the day before she died and that she was not sick. She described her daughter's marriage to Jason Harris as "rocky."

A neighbor said Jason Harris told them that the night before his wife died that he had given her cereal and milk and she dropped the bowl and passed out while eating, the prosecutor said.

His co-workers told detectives that prior to his wife's death, Jason Harris had said that he put five Xanax pills in his wife's water but that after tasting it she wouldn't drink any more. He had also hired a hitman to kill his wife but the man was arrested on an unrelated crime while he had Christina Harris under surveillance, Leyton said. Jason Harris then asked a coworker if they would kill his wife.

Jason Harris did not want to divorce his wife because he did not want to pay child support or alimony and feared his children would be taken away from him, the prosecutor said.

The investigation also revealed that Jason Harris had been contacting "numerous females" before and after his wife died, Leyton said.

Following the yearslong investigation, a medical examiner last week amended the cause of Christina Harris' death to homicide.

Jason Harris has been charged with premeditated murder, solicitation of murder and delivery of a controlled substance.

His lawyer told NBC News in a statement that he has pleaded not guilty to the charges against him "and looks forward to demonstrating his innocence in the unfortunate passing of his wife."

"Mr. Harris has been completely cooperative throughout this almost half-decade investigation and was disappointed to learn that the original findings of an accidental overdose have now been discarded," attorney Nicholas Robinson said.