The Alameda County District Attorney's Office is reviewing the allegations, which are detailed in a report the City Council made public after a closed session late Tuesday.
The city said that if the District Attorney concludes that Tam has engaged in misconduct, she could be removed from office and could face criminal charges.
Mayor Beverly Johnson, Vice Mayor Doug deHaan, Councilman Frank Matarrese and Councilwoman Marie Gilmore voted unanimously in the closed session to release to the public an investigatory report that details the alleged misconduct. Tam was asked to recuse herself from the closed session and didn't vote.
The evidence of alleged misconduct is largely found in emails between Tam and principals of SunCal, a developer negotiating with the city to develop the former naval air station, and the International Association of Firefighters union, the city said in a prepared statement.
The city said others who received confidential emails include John Knox White, a community activist who serves on the city's Sunshine Task Force, and community blogger Lauren Do.
City officials said the emails were collected over several months by Interim City Manager Ann Marie Gallant after she became aware of Tam's alleged use of the city's email system to initiate what the city believes are serial meetings that violate the state's open government law for local governments.
In February, more than 85 percent of Alameda voters rejected a proposal from SunCal to build about 4,500 new housing units and make other changes at the former base, now known as Alameda Point.
But SunCal still hopes to redevelop the site. Its exclusive negotiating deal with the city will expire July 20 unless the City Council extends it.
Tam has supported SunCal's efforts, but Mayor Johnson and other council members have opposed them.
The city's statement says Gallant and the city's attorneys brought their concerns about Tam to Johnson and the other council members Tuesday night because of "the importance of this decision" about developing the former naval base.
"This is a sad day for Councilmember Tam and the city of Alameda. We look to the district attorney and the grand jury to do justice in this matter," Johnson said.
But Tam said tonight that "there's nothing" to the allegations against her, and she expects to be "completely exonerated" after an independent investigation is conducted.
Tam said she doesn't think she has been violating the state's open meetings laws, and she believes Gallant is targeting her because "she finds it objectionable that I'm asking tough questions" about the city's policies.
The report was prepared by Michael Colantuono, an outside lawyer who was hired by the city to investigate Tam.