Law enforcement officials are continuing to search for Elizabeth "Lizzi" Marriott, but officials believe she is dead. Authorities said her car was found in a parking lot on campus in Durham.
Seth Mazzaglia, 29, was charged with second-degree murder, but Assistant Attorney General James Vara declined to say how the two knew each other or what led to his arrest.
A search has been under way on Peirce Island in Portsmouth, on the New Hampshire coast about 12 miles southeast of Dover, since Friday night.
The 19-year-old was last seen on Tuesday, and she texted a friend to say she would stop by another friend's house just after 10 p.m. but never arrived. Also, she never showed up at her job at Target on Wednesday and did not attend classes on Thursday.
Marriott's grandmother was tending the phone at her granddaughter's Chester home while other family members were out handing out fliers and searching for her.
"She is a good student and she's hard-working," her grandmother said. "She is a person that doesn't have a multitude of friends but she has good friends, a small group of good friends."
She said the FBI and other investigators were at the house for much of the day interviewing family members.
Marriott graduated Westborough High School in Massachusetts in 2011. She lived with an aunt in Chester and commuted to the Durham campus.
Mazzaglia, a University of New Hampshire graduate, was active in the arts, performing in dozens of plays and musicals over the last decade in southern New Hampshire. He was recently cast in a production by Garrison Players Arts Center in Rollinsford, N.H.
On his website, Mazzaglia described himself as having a black belt in Karate and being familiar with several forms of martial arts and weapons. He also described other interests, including juggling.
ABC News and The Associated Press contributed to this report.