Dinis was a resident of Sunrise Senior Living in Dartmouth, Mass., the Boston Globe reported. He was being treated for lymphoma.
The investigation in 1969 effectively ended Dinis's political career. He lost a bid for re-election as Bristol County district attorney in 1970 and never won another election.
"It was really the beginning of the end of his political career; he was between a rock and a hard place,'' said Henry Arruda, a lawyer and manager of the radio station Dinis owned in New Bedford.
Dinis angered Kennedy supporters by convening a grand jury to consider bringing charges against the senator. Kopechne drowned when Kennedy drove off a bridge on Chappaquiddick Island off Martha's Vineyard, and Kennedy detractors said the investigation was bungled.
The son of Portuguese immigrants from the Azores, Dinis started life with political credentials. His father served in the state legislature in the 1940s, and Dinis's godfather was the storied Boston politician James Michael Curley.
In his later years, Dinis owned WJFD-FM, New Bedford, a Portuguese-language station, and a motel.