GOSHEN, N.Y. — Her fiancé, separated from his kayak, struggled in the frigid and choppy waters of the Hudson River, reaching for his paddle to save his life. But prosecutors here in Orange County said on Tuesday that Angelika Graswald moved it away, and was slow to call for help.

In a news conference, prosecutors disclosed new details in the death of Vincent Viafore, the kayaker who had been missing for weeks; his body was found over the weekend in the river, not far from where he disappeared.

Prosecutors said Ms. Graswald, 35, set out to kill Mr. Viafore on April 19. She removed a plug from his kayak, they said, so that it would fill with water. She also tampered with his paddle. And though Mr. Viafore, 46, capsized around 7:15 p.m., it was not until about 7:40 that she summoned help.

The authorities have not disclosed the source of the details in their version of how events unfolded, but prosecutors did say that if the case goes to trial they plan to include as evidence written and oral statements from Ms. Graswald from the night of Mr. Viafore’s disappearance as well as from questioning by investigators the day before she was charged.