Montgomery County investigators seized a key document during their search this week of Pennsylvania Attorney General Kathleen G. Kane's offices that could undercut the defense in her criminal case, The Inquirer has learned.

As they executed a search warrant Thursday at Kane's Harrisburg office, the detectives found an oath of secrecy she signed shortly after taking office in January 2013. In the oath, she swore to keep confidential decades worth of grand jury investigations, according to sources familiar with the document.

The newly discovered oath could prove key in the case against Kane, who is charged with leaking secret grand jury materials to the Philadelphia Daily News last year in a bid to embarrass a political enemy, Frank Fina, a former top prosecutor in her office with whom she was feuding.

Kane and her lawyers have said repeatedly that she never took an oath of secrecy for the 2009 grand jury proceedings from which she is accused of leaking information. At the time, Kane was a stay-at-home mother, was not involved in that grand jury's proceedings, and cannot be held liable for the leak, her lawyers have argued.

But the oath investigators obtained shows Kane swore to hold secret information from Grand Jury No. 1 - impaneled 35 years ago - through Grand Jury No. 32, according to people familiar with the matter. The grand jury from which she is accused of leaking is Grand Jury No. 29.

Her criminal defense attorney, Gerald Shargel, has declined to discuss the search warrant.

On Friday, Shargel issued a statement calling for a formal investigation into "leaks" about Kane's criminal case and her disciplinary proceedings that he said are designed to unfairly taint potential jurors.

Shargel said Kane's legal team is growing "more frustrated and disappointed" by the day at media coverage of what should be confidential or protected matters involving her criminal case. The reports, he said, are based on leaks that threaten Kane's constitutional rights to hold elected office and to a fair trial.

"There are regular and unconscionable leaks of confidential information in this case," Shargel wrote in a statement. "An elected official is being railroaded out of office and forced to defend herself before a jury pool that has been intentionally and irrevocably tainted. This is absolutely and fundamentally wrong. It should not be allowed to stand."

Shargel asked the courts to launch an inquiry, calling it an "utter hypocrisy" that his client faces criminal charges for leaking.

Kane was charged last month with conspiracy, perjury, official oppression, and other crimes. The case is being led by Montgomery County District Attorney Risa Vetri Ferman and First Assistant Kevin Steele.

Officials with the Administrative Office of Pennsylvania Courts, the administrative arm of the state Supreme Court, did not have an immediate comment. The high court authorizes leak investigations.

In his statement, Shargel cited media coverage of the search warrant, saying that although it was sealed, reporters had clustered in the lobby of the Attorney General's Office as the search was unfolding, with some reporting on its contents.

Shargel also cited in his statement media coverage of an emergency petition by the state Disciplinary Board to the Supreme Court to suspend Kane's law license. If the justices opt for suspension, Kane could be forced from office, as the state Constitution requires the attorney general to be a member of the bar.

Such disciplinary proceedings are confidential, as are all the documents about it. Shargel contended that the press had one of the disciplinary documents even before Kane or her attorneys. He did not specify a media outlet.