Then-FBI Director Clarence M. Kelley relayed Cox's concern in a memo on Aug. 16, 1973, to the bureau's Cleveland office, saying agents needed to make sure the probe received "the same, immediate and preferred handling" as other criminal cases then growing from the Watergate scandal.
The memos were included in a 400-page release of the late Steinbrenner's FBI file Thursday. Most of the material focused on the Watergate-era federal probe that led to the shipbuilding magnate's 1974 conviction for illegal contributions to disgraced President Richard M. Nixon.
The FBI said it was an interim release and that more documents would be forthcoming at a later date.
The Yankees declined to comment on the FBI file.
The Associated Press and other news organizations requested the file under the Freedom of Information Act following Steinbrenner's death in July.
"The office of the Special Prosecutor has indicated extreme interest in this matter and requests that the interviews be conducted as soon as possible, and as nearly at the same time as possible," Kelley wrote in the memo on the investigation into Steinbrenner and his Cleveland-based American Ship Building Co.
Another FBI memo, dated Oct. 17, 1973, says the "investigation is to be afforded highest priority and security. Cleveland office) to assign most capable personnel to achieve prompt positive results."
Among other things, the FBI was investigating whether employees were told they would be reimbursed by the company for campaign contributions, a violation of campaign finance laws.
Steinbrenner was indicted the following year and vowed to prove his innocence in court. In August 1974, two weeks after Nixon resigned, the Yankees owner pleaded guilty to two charges in the case and was fined $15,000.
American Ship Building Co. executives told FBI officials in signed statements that they received bonuses around the same time they made donations to Nixon's campaign. The payments varied from $2,800 to $3,500, according to a report on Aug. 30, 1973, by the Cleveland office of the FBI.
The employees claimed the donations "were all of their own desires and in no way were motivated or solicited by George M. Steinbrenner or any other company officials," the report said. "Those interviewed stated that they would have made contributions to the Nixon campaign regardless of the receipt of their bonuses."
The FBI seemed to doubt those claims, with a later memo referring to "the fabrication of the deceptive cover story told by the company officials during their interviews."
One charge that Steinbrenner later pleaded guilty to involved a conspiracy to funnel corporate campaign contributions to politicians. The other accused Steinbrenner of making a "false and misleading" explanation of a $25,000 donation to Nixon's campaign and trying to influence and intimidate employees of his shipbuilding company to give that false information to a grand jury. Steinbrenner could have faced up to six years in prison for the guilty pleas but was not sentenced to jail time.
The memos also document several times the FBI tried unsuccessfully to interview Steinbrenner, only to be told he was traveling.
Steinbrenner bought the Yankees in 1973, but baseball commissioner Bowie Kuhn suspended him for two years after his 1974 plea, calling him "ineligible and incompetent" to have any connection with a baseball team.
"Attempting to influence employees to behave dishonestly is the kind of conduct which, if ignored by baseball, would undermine the public's confidence in our game," Kuhn wrote in a 12-page ruling. The suspension was later reduced to 15 months.
Preisent Ronald Reagan pardoned Steinbrenner at the end of his final term in office.
Steinbrenner told the New York Times in 1988 that he felt bad about the conviction and was willing to bear responsibility for the crimes, but said they happened because he didn't understand what the campaign finance act required of him.