On the night of July 18, 1969, Kennedy was on Martha's Vineyard's Chappaquiddick Island at a party for the "Boiler Room Girls", a group of young women who had worked on his brother Robert's presidential campaign the year before. Leaving the party, Kennedy was driving a 1967 Oldsmobile Delmont 88 with one of the women, 28-year-old Mary Jo Kopechne, as his passenger, when Kennedy drove off Dike Bridge into Poucha Pond between Chappaquiddick Island and Cape Poge barrier beach. Kennedy escaped the overturned vehicle and swam to safety, but Kopechne died in the car. Kennedy left the scene and did not call authorities until after Kopechne's body was discovered the following day.
On July 25, Kennedy pleaded guilty to leaving the scene of an accident and was given a sentence of two months in jail, suspended. That night, Kennedy gave a national broadcast in which he said, "I regard as indefensible the fact that I did not report the accident to the police immediately," but denied driving under the influence of alcohol and denied any immoral conduct between him and Kopechne. Kennedy asked the Massachusetts electorate whether he should stay in office, and after getting a favorable response, he did.
In January 1970, an inquest into Kopechne's death took place in Edgartown, Massachusetts. At the request of Kennedy's lawyers, the Massachusetts Supreme Judicial Court ordered the inquest be conducted in secret.Judge James A. Boyle presided over the inquest. His conclusions were as follows:
"Kopechne and Kennedy did not intend to return to Edgartown" at the time they left the party.
"Kennedy did not intend to drive to the ferry slip".
"[Kennedy]'s turn onto Dike Road was intentional".
Judge Boyle also said that "negligent driving appears to have contributed to the death of Mary Jo Kopechne." Under Massachusetts law, Boyle could have ordered Kennedy's arrest, but he chose not to do so. A grand jury on Martha's Vineyard staged a two-day investigation in April 1970 but issued no indictment, after which Boyle made his inquest report public. Kennedy deemed its conclusions "not justified." Doubts about the Chappaquiddick incident generated a large number of articles and books over the next several years.