PHILADELPHIA For a few months in 1966, the budding romance between film star George Hamilton and Lynda Bird Johnson, daughter of the 36th president, was the talk of Washington.

Gossip columnists followed their every move as Hamilton squired her around town. The couple vacationed in Acapulco and made camera-ready appearances at the Sugar Bowl, Mardi Gras and the Oscars. The actor spent Easter at the LBJ ranch in Texas and even attended the Washington wedding of Lynda's sister Luci.

President Lyndon Johnson made no secret of his suspicions about the handsome, patent-leather playboy, perhaps best known now for his perpetual tan, blinding smile and roles on TV shows ranging from "Columbo" to "Dynasty" to "Dancing with the Stars."

But a previously confidential FBI file which a Philadelphia judge last week outlined in an opinion and ordered to be released shows for the first time how far Johnson went to protect his daughter and his presidency.

The file indicates Johnson enlisted Supreme Court Justice Abe Fortas and J. Edgar Hoover's FBI to investigate every rumor they could find about Hamilton, including claims that he was gay and a draft-dodger, in a bid to dig up dirt on the actor.

In his ruling, U.S. District Judge Eduardo Robreno called it not only an improper probe but a "potentially illegal use of executive power."

The documents were the focus of a four-year court battle by a Villanova Law School professor, Tuan Samahon, and his students. But they also offer a window into a presidential administration and an FBI that apparently thought little of violating the privacy of American citizens an accusation that has resonated for modern presidential administrations.

According to Robreno, who reviewed the controversial file, the documents ended up reflecting most poorly on the FBI itself.

"This case is about the ability of the federal government to pry into the private lives of U.S. citizens with virtual impunity," he wrote in his opinion. "The file can be read as an effort by the FBI to uncover embarrassing details about a private citizen as a personal favor to the president."

The FBI file burnishes a long-established record of the excesses of Hoover's agency and Johnson's willingness to use it to investigate perceived threats. But that wasn't what Samahon, who teaches courses on constitutional law and federal courts, initially went looking for.

He wanted to know what role the FBI may have played in the 1969 resignation of Fortas from the highest court after only four years. Fortas, a Johnson appointee to the court, had been the president's former attorney and longtime confidant.

Samahon filed a Freedom of Information Act request in 2010 to see a memo that he hoped would give him material for a book on Fortas. At the time, he believed it could indicate the FBI used knowledge of some illicit relationship Fortas had with a man to pressure him into disclosing confidential information about a Supreme Court case.

The Department of Justice released the memo but redacted a single name, saying it could reveal embarrassing details about a private citizen.

Samahon rejected the argument, saying there was no legal reason to keep the name confidential, but the FBI didn't budge. So Samahon put his students to work, and in 2012 sued for the documents' release, as well as for the release of the file containing the memo. Samahon said 19 students and Beth Lyon, another Villanova professor, devoted many hours to the case over two years.

The memo Samahon wanted was a two-page report by Cartha DeLoach, deputy director of the FBI and Hoover's right-hand man.

DeLoach, then the third-highest-ranking official in the FBI, had investigated some of the nation's most notorious crimes, including the assassinations of President John F. Kennedy and the Rev. Dr. Martin Luther King Jr. He was a Hoover loyalist with close ties to Johnson, and many believed he regularly leaked information to the White House about the most salacious FBI investigations.

As the romance blossomed between Hamilton and the president's daughter in early 1966, DeLoach and Fortas were given the uncommon task of sabotaging the relationship. The president, DeLoach wrote in his memoir, also wanted "a full rundown" on Hamilton.

"As far as the president was concerned, Fortas' seat on the Supreme Court didn't preclude him from doing a little moonlighting for the president," DeLoach wrote.

DeLoach and Fortas had a laugh over it, according to DeLoach, then began what DeLoach called a "discreet background check," reviewing the actor's family, friends, credit history, draft deferment and more.