Angelika Graswald being arraigned in State Supreme Court in Orange County, N.Y., in May 2015, on charges that she killed her fiancé, Vincent Viafore. She pleaded guilty on Monday to criminally negligent homicide.

A woman accused of killing her fiancé by tampering with his kayak and then leaving him to drown in the cold and choppy waters of the Hudson River pleaded guilty on Monday to a reduced charge of criminally negligent homicide.

The case drew attention far beyond the area just north of New York City where the death occurred, as prosecutors described how a seemingly doting partner desperately called 911 for help after her fiancé disappeared only to tell investigators later that it “felt good knowing he was going to die.”

The woman, Angelika Graswald, 37, was awaiting trial for second-degree murder and faced a maximum sentence of life in prison. She was charged in April 2015, after her fiancé, Vincent Viafore, 46, vanished after his kayak capsized during a trip on the Hudson River. The authorities said that Ms. Graswald had removed a plug from the boat and knew that a locking clip that kept the paddle in place was missing.

The couple was returning on the evening of April 19 to the western shore of the Hudson from Bannerman Island, several miles north of West Point. The wind picked up, and Mr. Viafore, who was not wearing a life jacket or a wet suit, fell into the water. (His body was recovered in May.)

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In the days after Mr. Viafore’s disappearance, Ms. Graswald behaved in ways that struck some who knew the couple as unusual, singing “Hotel California” at a local pub and using social media to post selfies and a video clip that showed her doing a cartwheel. She was arrested on April 30, a day after an encounter with investigators on the island she had visited with Mr. Viafore the day he disappeared. She had returned there to leave flowers in his honor.

Prosecutors in Orange County, N.Y., said Ms. Graswald had made conflicting statements that implicated her during a lengthy interrogation with investigators and that she knew she was the beneficiary of two life insurance policies belonging to Mr. Viafore and stood to receive a total of $250,000. Her lawyer, Richard A. Portale, argued that she was coerced by investigators and that there was a language barrier between Ms. Graswald, a native of Latvia, and the police. He also said she was unaware of her rights, at one point asking, “Who’s Miranda?”

Ms. Graswald’s trial was to begin next month after multiple delays, and Mr. Portale said that prosecutors had approached them this month with the plea offer.

Image Bannerman Castle on Pollepel Island in the Hudson River, near where Mr. Viafore died while kayaking in April 2015. Credit... Robert Stolarik for The New York Times

“This plea ensures that the defendant will be held criminally liable for her actions,” David M. Hoovler, the Orange County district attorney, said in a statement on Monday. “By pleading guilty the defendant has acknowledged that Vincent Viafore’s death was not simply a tragic accident, but the result of this defendant’s criminal conduct.”

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Ms. Graswald is scheduled to be sentenced on Nov. 1. Under the terms of the agreement, she is expected to receive from 15 months to four years in prison, prosecutors said. But Mr. Portale said that, taking into account the 27 months she has spent in jail since being arrested, she expects to be released by the end of the year.

“She knows she’s coming home,” Mr. Portale said. “She can focus on moving forward.”

By pleading guilty to criminally negligent homicide, Mr. Portale said, Ms. Graswald acknowledges that she took the plug from the kayak and that she was aware of the rough weather conditions on the river that day, but not that she intended to kill Mr. Viafore. “The record is clear that this was not an intentional killing,” he said. “This was a case of negligence.”

Ms. Graswald came to the United States from Latvia when she was 20, taking a job as an au pair for a family in Connecticut. Mr. Viafore, a construction manager who lived in Poughkeepsie, grew up near the Hudson River, in Wappinger, in southern Dutchess County. Both had been married twice before.

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Friends had described them as a loving couple who traveled to Eastern Europe together. On Facebook, Mr. Viafore had posted a photo of the couple kissing in front of a setting sun, saying they planned to have their wedding on the Baltic Sea.

But during the interrogation, a video recording of which was expected to be presented as evidence at trial, Ms. Graswald expressed frustration to investigators about the couple’s relationship, complaining about what she said was Mr. Viafore’s interest in pornography and group sex. She also described her ambivalence as he drowned.

At the end of the questioning, she blurted out, “I wanted him dead and now he’s gone and I’m fine with it.” But in a subsequent television interview, Ms. Graswald said of Mr. Viafore that she “loved him” and “didn’t do anything to kill him.”

Prosecutors said that in addition to removing the kayak’s plug, Ms. Graswald had also tampered with Mr. Viafore’s paddle. After he fell into the water, prosecutors said, she delayed in calling 911 and intentionally capsized her own kayak to make it look as if she had tried to save him.

After Mr. Viafore’s body was found in the river weeks later, the medical examiner, in a decision that was met with criticism, was explicit in detailing the cause of death: “kayak drain plug intentionally removed by other.”

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