A federal appeals court has ruled that a former Miami Dolphins coach—accused of bullying a player—did not have a valid defamation claim against a law firm that investigated the team's locker room culture.

The decision has significant implications for companies that engage in internal investigations. It also is the latest in an emerging line of cases dismissing public-figure defamation claims early when a plaintiff fails to plead a plausible claim of First Amendment "actual malice."

The Miami Dolphins had well-publicized locker room issues from 2012 through 2014 involving allegations of player bullying. These issues culminated in second-year lineman Jonathan Martin deciding to leave the team early in the 2013 season and seek psychiatric help.

Following intense media scrutiny, the Dolphins retained a law firm to conduct an internal investigation into its locker room culture. In a 144-page report, the firm found widespread bullying—including multiple instances of homophobic bullying—and singled out an assistant coach, James Turner.

Mr. Turner sued the law firm that prepared the report, Paul, Weiss, Rifkind, Wharton & Garrison LLP, for defamation. A federal district court in Miami dismissed Turner's defamation claims against the law firm and the lead report author, Theodore Wells.

In affirming dismissal last month, the U.S. Court of Appeals for the 11th Circuit held that the report's conclusions about Mr. Turner were protected opinions, as they were conclusions based on admittedly true facts stated in the report.

Significantly, the court held that Mr. Turner was a public figure who "chose to put himself in the public arena" by appearing in the 2012 season of Hard Knocks on HBO, engaging with the media following the law firm's report, and texting Mr. Martin to try to persuade him to make a public statement. In a ruling that adds helpful precedent for defending public figure defamation claims, the court further held that Mr. Turner failed to plead a plausible claim for First Amendment actual malice—that the law firm knew the statements were false or subjectively entertained doubts as to their truth—because he did not allege facts showing that the defendants "knowingly or with reckless disregard" published false statements. Instead, the court held, Mr. Turner attempted to base his malice claim on failure to include additional facts that were either contained in the report or were deemed immaterial, and on conclusory statements of malice that must be disregarded under federal pleading standards that require plaintiffs to state facts that plausibly support their claims.