CONCORD, N.H. — A man charged with killing a University of New Hampshire student in October plotted with a girlfriend to tell investigators she was strangled during a consensual sex act, prosecutors said.

Court documents released Tuesday charge 30-year-old Seth Mazzaglia of Dover with killing 19-year-old Elizabeth “Lizzi” Marriott during an attempted sexual assault on Oct. 9.

The indictments allege an extensive cover-up by Mazzaglia and his then-girlfriend, Kathryn McDonough, including using Marriott’s car to drive her body to Peirce Island in Portsmouth and dumping it in the Piscataqua River. Her body has never been recovered.

Mazzaglia is also charged with second-degree murder for showing “extreme indifference to human life.”

A hindering prosecution charge details a series of actions the couple allegedly took to mislead police. Prosecutors say McDonough sent a text to Marriott an hour after she was killed at their Dover apartment asking why she never showed up for their planned visit.

The indictments allege the pair also broke Marriott’s cellphone and GPS tracking system and abandoned Marriott’s car in a parking lot at the University of New Hampshire, where Marriott had transferred to as a sophomore to study marine biology. Prosecutors say they also concocted the plan to say Marriott was killed accidentally during a consensual sex act — a notion that the lead prosecutor and Marriott’s family Tuesday decried as false.

“There’s no allegation of anything consensual,” Senior Assistant Attorney General Jeffery Strelzin said Tuesday. “On the contrary, it was not consensual.”

Marriott’s parents, Bob and Melissa Marriott of Westborough, Mass., called the suggestion their daughter was killed during some bizarre consensual sex act “reprehensible.”

Their lawyer, George Thompson, said the Marriotts are “resolute in their demands for justice for Lizzi” and are satisfied with the indictments.

Mazzaglia faces two alternate theory first-degree murder charges. One alleges he strangled Marriott. The other alleges he used physical force to strangle her before, during or after an attempt to sexually assault her.

Marriott’s disappearance on Oct. 9 led to a massive search, and her family and friends distributed leaflets throughout the seacoast region seeking clues to her whereabouts. Her cellphone was last used in Dover at 10:11 p.m. that night.

On Oct. 13, four days after Marriott was last heard from, police took Mazzaglia into custody and charged him with second-degree murder.

An outdoor memorial service at UNH drew hundreds of somber students, some of whom spoke of exuberance and love of life.

University President Mark Huddleston told the sea of students that even though she had been at UNH only a short time, she was very much a part of the college community.

“Losing Lizzi was losing part of ourselves,” Huddleston said.

Mazzaglia, who is being held without bond, is due in court May 2. Attorney Joachim Barth, one of the public defenders representing Mazzaglia, on Tuesday declined to comment on the indictments.

McDonough is due to return to court Thursday for a bail hearing, but may waive that hearing. She was indicted earlier this month on charges of hindering the prosecution, conspiracy and witness tampering.