"We're hoping for more as we go through the day," he said.
Steinberg, D-Sacramento, said he had no comment and did not know anything about the investigation.
Officers from the California Highway Patrol and Senate sergeant-at-arms details were standing guard outside the office, where a morning newspaper remained untouched.
Yee, a Democrat, represents western San Francisco and much of San Mateo County. A spokesman for the senator, Dan Lieberman, said he had no comment, but the senator's office would release a statement in the afternoon.
Yee, 65, is best known publicly for his efforts to strengthen open records, government transparency and whistleblower protection laws, including legislation to close a loophole in state public records laws after the CSU Stanislaus Foundation refused to release its $75,000 speaking contract with former vice presidential candidate Sarah Palin in 2010.
For his efforts to uphold the California Public Records Act, Yee was honored last week by the Northern California chapter of the Society of Professional Journalists, which awarded him its public official citation for his efforts last year to maintain the requirements of the California Public Records Act.
Yee has at times clashed with fellow Democrats for casting votes of conscience, refusing to support the Democratic budget proposal in 2011 because of its deep cuts to education, social services and education. He also opposed legislation by a fellow Democrat, Assemblyman Paul Fong of Cupertino, that banned the sale of shark fins used for Chinese shark fin soup, saying that it unfairly targeted the Chinese-American community.
Yee is among three Democrats running this year for secretary of state, the office that oversees elections and campaign finance reporting. He lost a bid for mayor of San Francisco in 2011.
A man was charged last year for threatening Yee over legislation that he proposed to limit rapid reloading of assault weapons.
Yee is scheduled to be arraigned at 1:30 p.m. in federal court in San Francisco.
Associated Press writers Terry Collins, Garance Burke and Jason Dearen in San Francisco; and Judy Lin and Juliet Williams in Sacramento, Calif., contributed to this report.