CHAPPAQUIDDICK TRAGEDY: Kennedy’s Second Passenger Revealed. Donald Frederick Nelson. Pelican. 191 pages, hardcover, sold in bookstores and online.

A new book by a Vineyard author tackles an old controversy. Anyone interested in the 1969 Chappaquiddick tragedy will find it compelling reading.

A car driven by U.S. Senator Edward M. Kennedy of Massachusetts plunged off Chappaquiddick Island’s Dike Bridge and submerged in Poucha Pond at approximately 12:45 a.m. on July 19, 1969. Mr. Kennedy escaped the vehicle, but a passenger named Mary Jo Kopechne did not. Mr. Kennedy said he dove repeatedly and unsuccessfully into the water, attempting to rescue Miss Kopechne. Then, he said, he walked 1.2 miles (past two lighted homes and a firehouse) back to Chappaquiddick’s Lawrence cottage, where he had been partying with political associates and six young Kennedy acolytes known as the “boiler room girls.”

Later, according to his testimony, because the On Time ferry ceased operations to Edgartown around midnight, the senator flung himself into Edgartown Harbor’s outgoing tide, crossed to a beach near the Edgartown lighthouse and walked back to the Shiretown Inn where he was staying. In the aftermath, which is painstakingly detailed in the book, accommodating police, politicians and attorneys general did a slapdash job of investigation, and Mr. Kennedy got off with a brief license suspension and a suspended two-month jail sentence for leaving the scene of an accident. None of the investigators uncovered, though it was on the public record, the fact that as a law student at the University of Virginia, Mr. Kennedy had been convicted of three moving violations. The information wasn’t disclosed until the morning Mr. Kennedy pleaded guilty.

Suspicions grew immediately. For one thing, back problems dating to a plane crash he’d survived several years earlier raised eyebrows about the desperate swimming he described. Mr. Kennedy had competed in the Edgartown Yacht Club sailing regatta on July 18 and subsequently took a bath to ease his pain. Was he in any position to dive into dark waters off the bridge and later fight the harbor’s current (which was actually incoming), even though a phone line at the dock could have summoned the ferry after hours? And since Miss Kopechne might have been breathing in an air pocket at the back of the car, why did he bypass two houses and a fire station and fail to report the accident until nine hours later? Moreover, if he was aware of her predicament, why was he chatting amiably with fellow guests at the inn hours later?

The truth, says author Donald Frederick Nelson, is that Senator Kennedy didn’t learn about Miss Kopechne’s fate until he subsequently received the news by phone from his cousin, Joseph A. Gargan, and former U.S. Attorney Paul F. Markham.

There have been dozens of books, studies, television investigations, blogs, rumors and innuendo about the tragedy. A few weeks ago, a Hollywood film crew arrived to shoot scenes for a major motion picture. And now we have an exhaustingly researched book by a retired, award-winning research physicist who lives in Worcester and Oak Bluffs with his wife Margaret. I know Mr. Nelson, 86, from Island bridge clubs and find him sober-sided and fact-oriented. By his account, he consulted 204 sources and read through thousands of pages of legal proceedings. His writing is as lyrical as it is factual.

The author reasonably asks why Mr. Kennedy didn’t seek help immediately and concludes that he didn’t realize there was a passenger in the back seat of his Oldsmobile 88. Mr. Nelson writes that Miss Kopechne, who had imbibed three or four alcoholic drinks in her last hour and probably more during the four-hour party, according to later blood work, had crawled into the back seat of the car to sleep the alcohol off. And then Mr. Nelson delivers a crushing blow to the Kennedy account that had been speculated for years but probably never given such exhaustive proof. He contends there was a another passenger, Rosemary (Cricket) Keough, sitting in the passenger seat and who successfully exited the vehicle after its plunge. Neither she nor Mr. Kennedy noticed the dozing woman behind them. Since there was apparently no tragedy, there was no hurry to report the incident.

“Kennedy was guilty of reckless driving and lying repeatedly about the wreck, but he was far from being so callous as to knowingly walk away from a trapped, dying person,” Mr. Nelson writes.

The author builds an impressive case for Mr. Kennedy’s additional passenger. Miss Keough’s handbag was found in the front of the car. If another woman had entered the vehicle to drive off with the senator, surely she’d have alerted Miss Keough and returned the bag to her, Mr. Nelson suggests.

Ms. Keough claimed to have left the handbag from an earlier drive, a supposition disputed by a next-door neighbor who had been watching the raucous party. Miss Keough wasn’t grilled sharply at the inquest. Questioning the politician on the morning of July 19, Edgartown police chief Dominick J. Arena asked: “Can you tell me, was there anybody else in the car?”

“Yes,” said Mr. Kennedy.

But Chief Arena never followed up on that disclosure. When Ms. Keough visited Mary Jo’s grave on the first anniversary of her death, she said: “My friend Mary Jo just happened to be in the wrong car at the wrong time with the wrong people.”

Miss Keough repeated those exact words to the Boston Globe Spotlight team of investigatory reporters five years later. The author notes that “people” is a plural noun.

Unfortunately, one piece of clinching evidence is still missing. Mr. Nelson writes of Mr. Kennedy: “His companion, sitting in the front passenger seat, was thrown violently against the right front door and window and then against the ceiling, as the car struck the water first on its right side and then its roof. Upon impact, shards of sharp, tempered glass from her shattered side window struck her head, shoulders and right arm. There were no restraining seatbelts. Lacerations, abrasions and contusions on the face, head, arm and shoulder seem inevitable for a front, passenger-seat occupant. But there were no such injuries whatsoever to Mary Jo!”

Were there signs of injury on Ms. Keough? She called Chief Arena asking for her handbag, and he told her to come to the station. But she sent someone else and was quickly whisked off the Vineyard. Ms. Keough did not answer phone messages left by the author.

Mr. Nelson offers an ironic afterthought. If Mr. Kennedy had come clean about everything from the beginning, he might have escaped with nothing more than a license suspension and been free to pursue the Democratic Party nomination for president in 1972. As Richard Nixon, the sitting president at the time, would discover, sometimes the cover-up is worse than the crime.

Jim Kaplan is the Gazette’s bridge columnist and the author of Clearing the Bases: A Veteran Sportswriter on the National Pastime.