A North Dakota woman was charged with murder after authorities say she poisoned her long-time boyfriend with antifreeze after learning he would receive a multimillion-dollar inheritance and that he planned to break up with her.

Police in Minot, North Dakota, said Steven Edward Riley, 51, died Sept. 5, and they believe his girlfriend, Ina Thea Kenoyer, 47, "had financial motives to murder Riley." Kenoyer was arrested Monday and is being held at the Ward County Jail, police said.

Riley died by poisoning, an autopsy determined.

Friends say man became ill while meeting with lawyer about inheritance

Riley started showing signs of illness on Sept. 3, the day he, Kenoyer and friends were at an airport so he could meet with an attorney about the inheritance he was set to receive, which Kenoyer said she believed to be over $30 million, according to an arrest warrant affidavit written by Minot Police Officer Erin Bonney and provided to USA TODAY.

Riley was listed as "John Doe" in the document but was identified by Minot police in a news release on Monday.

Witnesses told investigators Riley's health "declined rapidly," according to the affidavit. He said he felt like he was drunk, but hadn't had any alcohol, and had stomach pain and was unsteady on his feet, the witnesses said. Riley's friends said they wanted to take him for medical treatment, but Kenoyer insisted he just needed to go home and rest and was suffering from heat stroke, the affidavit states.

Within an hour, he was vomiting and appeared to be losing consciousness. The next morning, a friend went by the house to check on Riley and told investigators that Kenoyer would only speak to him through a closed screen door and said Riley had gone to a walk-in clinic. The friend said he went to every clinic and emergency room in the area but could not find Riley, according to the affidavit.

At about noon on Sept. 4, an ambulance responded to a 911 call that a man was unconscious. Riley was taken to a nearby emergency room but had to be flown to a medical center in Bismarck because his condition was so severe, the arrest warrant said. He never regained consciousness and died the next day, according to the affidavit.

Woman made comments about poisoning boyfriend with antifreeze, witnesses say

After Riley's death, friends and family told police they thought Kenoyer might have poisoned him, the affidavit states. They said she had made comments before and after his death about poisoning him with antifreeze, though no test had been done to detect it at the time, the warrant says.

A friend also reported seeing Kenoyer throw Riley's belongings outside the afternoon he met with the lawyer about the inheritance. Friends said Riley was planning on breaking up with Kenoyer after receiving the money, according to the affidavit. They had dated for about 10 years.

The coroner tested for antifreeze after learning about their concerns and found "toxic" levels of ethylene glycol, a key component of antifreeze, in his system, the affidavit said; he had no alcohol in his system. Authorities also found an old Windex bottle with bright green fluid suspected to be antifreeze, and a glass beer bottle and a mug with suspected antifreeze in them.

Investigators said Kenoyer admitted to serving sweet tea to Riley throughout the day on Sept. 3. The affidavit also notes that antifreeze is easy to disguise in sweet drinks.

Suspect was 'incensed' when told she wouldn't receive any of the money

Kenoyer told detectives that she planned to split the inheritance money with Riley's son because she believed she was entitled to half as Riley's "common law wife," according to the affidavit.

"Kenoyer was incensed when appraised by detectives that North Dakota does not recognize common law marriage and she would in fact not be entitled to a portion of any inheritance," the affidavit states.

It wasn't clear whether Kenoyer had an attorney who could speak on her behalf, and court records indicate she is representing herself. She was charged with class AA felony murder, the state's most severe charge, which carries a maximum sentence of life in prison without parole.