SAN JOSE -- After getting a tip from the San Jose city clerk about the possibility of an additional forged signature on the messy paper trail of former Supervisor George Shirakawa Jr., local prosecutors on Monday broadened their investigation into the now disgraced politician.

If proved a forgery, it would mean his deception of voters spanned his races for county supervisor, a local school board, as well as one of two terms as a San Jose city councilman.

Shirakawa, a Democrat, resigned March 1, the day he was charged with multiple felony counts for perjury, accepting campaign donations in cash and using his county credit card for his personal entertainment. He has agreed to plead guilty March 18 to all the charges, including filing campaign finance reports between 2005 and 2012 with the forged signature of his longtime friend, Paz Rocha.

The San Jose city clerk uncovered an odd, mismatched signature when comparing Shirakawa's Form 460 dated July, 31, 2001, to earlier versions. The District Attorney's Office is reviewing the newly surfaced documents.

"I called the DA because when somebody signs under penalty of perjury and it appears that they may have lied, then I need to get the DA involved," San Jose's acting City Clerk Toni Taber said Monday. "There are irregular signatures. They don't look the same; in fact, they look substantially different."

Shirakawa could not be reached for comment on Monday.

Shirakawa appears to have had several treasurers during the course of his two successful City Council campaigns. He was elected in 1994 and again in 1998. The treasurers' names include Gary Clark and a series of family members, including his younger sister Navette Shirakawa Heberling and Steven Heberling; and his younger brother Kenley Shirakawa. Clark had signed on as treasurer in May 1997 and appears to have signed numerous campaign reports in a similar fashion thereafter, until the suspect signature in July 2001.

Public records indicate that a Gary Dean Clark, born in March 1961, lived at addresses listed on the campaign filings over the years. The Mercury News was unable to reach Clark on Monday.

Taber said she's not an expert on the authenticity of signatures, but with Clark's signature in particular, there was a clear irregularity. His original signature on the campaign's statement of organization, or Form 410, was "big and loopy." But the signature on July 31, 2001, "looked like someone could be writing with their off hand, or trying to trace," Taber said. "It's not one fluid movement; it looked like this person had paused. The signature is segmented."

Taber said candidates and officeholders must file accurate reports in order to earn and keep the public's trust. "It's unethical," to forge signatures, she said. "It's a lie, and you don't want your elected officials to lie."

Assistant District Attorney Karyn Sinunu-Towery confirmed that her office received a tip Monday morning "that there was a problem with the City Council campaign forms." Sinunu-Towery said she could not verify that a crime had taken place with regard to the San Jose campaign forms until the investigation was completed, but added "we're going to look into that, obviously." It's unclear whether the latest information will affect Shirakawa's case.

Shirakawa already has admitted to filing documents over seven years with the fake signatures of Rocha, his longtime friend and former co-coach at Yerba Buena High School. Rocha learned of the deception in January, when an investigator with the District Attorney's Office questioned him about his signatures on at least 15 of Shirakawa's campaign filings in his race for the East Side Union High School District board. Those campaign finance reports are at the center of the DA's case proving that Shirakawa used $130,000 in campaign and taxpayer funds to gamble at California and Nevada casinos. Cora Tomalinas, a community activist and longtime Shirakawa supporter, said she was dismayed to read about the Rocha forgeries in a Mercury News story Sunday. "I didn't know any of this, that's what makes me feel worse," she said. "I didn't see any of this coming."

For her part, Taber proactively sought Shirakawa's campaign forms, which had been in the city's Corporation Yard warehouse on Senter Road, where archived documents are kept. She did so in anticipation of requests for the documents, which she assumed would surface after the charges against Shirakawa were filed March 1.

Contact Karen de Sá at 408-920-5781. Contact Tracy Seipel at 408 275-0140.