A Dartmouth College sorority and two fraternity members have been charged with underage alcohol offenses in the death of a 20-year-old student who drowned after attending an off-campus party this summer.

The authorities said the student, Won Jang, attended the party hosted by the sorority Alpha Phi on July 6. Most people at the gathering, including Mr. Jang, were underage and drank alcohol that was provided by members of Mr. Jang’s fraternity, Beta Alpha Omega.

Alpha Phi was charged with a misdemeanor for hosting a party where underage drinking occurred, according to a Friday news release from the police department in Hanover, N.H., where Dartmouth’s campus is. Two members of Beta Alpha Omega, who are not underage, were each charged with a misdemeanor for supplying alcohol to attendees under 21, the release said.

On the night of the party, several attendees went swimming in the Connecticut River, which runs along Hanover, the police said. Many departed when a heavy rainstorm hit, but Mr. Jang, who his family said could not swim, was left behind.

Mr. Jang was found dead in the river the day after the party, the authorities said. The medical examiner’s office determined that the cause of death was drowning, and a toxicology report found that Mr. Jang had a blood alcohol level that indicated he was likely to have been significantly impaired.

Lt. Michael Schibuola of the Hanover Police Department said the police had investigated whether hazing had contributed to Mr. Jang’s death but ultimately determined it had not.