News organizations requested the FBI file under the Freedom of Information Act after Steinbrenner's July death. The first release was made in December. The two releases totaled about 800 pages.
In a newly released 1988 FBI memo, the FBI said it "supports the contention that George Steinbrenner has provided the FBI with valuable assistance."
Seven months later, Reagan pardoned Steinbrenner for convictions in a case involving campaign donations to President Nixon and other politicians.
The documents, included in the second release of Steinbrenner's FBI file, show he blamed his illegal corporate campaign contribution to Nixon on bad legal advice.
The memo disclosed Monday described one investigation in which Steinbrenner assisted "an undercover operation" that led to a conviction. The FBI described the other investigation simply as "a sensitive security matter." It deleted all specifics about the investigations before releasing the file.
A separate FBI document identifies the cases as "two national security matters" and says Steinbrenner assisted the bureau from 1978 to 1983.
A 1987 letter by Steinbrenner's lawyers about his assistance to the FBI says he "knows that he placed the lives of his family and himself in jeopardy through being involved in a terrorist matter."
The 1988 FBI memo says Steinbrenner agreed to use Yankee Stadium to stage more than 500 gambling raids against a major organized crime syndicate in New York City. A different site was ultimately chosen.
Steinbrenner pleaded guilty in 1974 to a conspiracy to funnel corporate campaign contributions to politicians. He also pleaded guilty to 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.
Five years later, Steinbrenner sought a pardon.
"Applicant advised that this corporate contribution was made after he received legal advice from corporate counsel, both inside and outside (Steinbrenner's) American Shipbuilding Company, that this corporate contribution was legal," stated a 1979 FBI memo. The memo quotes Steinbrenner as saying he wouldn't have made the contribution had he known it was illegal, and that his lawyers should have been more thorough in their research.
Steinbrenner also said he never told any employee to lie about the corporate donation, nor suggest they should repeat his version of the facts.