SAN DIEGO -- Police are investigating a series of threatening letters sent to three San Diego City Councilmen indicted in August on corruption charges, it was reported Friday.

Councilmen Charles Lewis, Ralph Inzunza and Michael Zucchet began receiving death and sexual-assault threats by mail at their downtown offices Sept. 3, according to court documents obtained by The San Diego Union-Tribune.

The computer-generated one-page letters contained various messages, some of them sexually graphic.

Police suspect the person behind the letters also may be responsible for a series of e-mail posters sent to about 20 city officials in May, all "clearly intended to degrade " Zucchet, according to a search warrant affidavit obtained by the Union-Tribune.

The e-mail posters were sent May 28 to Mayor Dick Murphy, his chief of staff, John Kern, City Attorney Casey Gwinn, and other senior city employees.

Kern e-mailed the sender and asked that no more of this type of e-mails be sent, according to the Union-Tribune. A reply came later that day: "The truth hurts, doesn't it?"

Police were not called until the more explicit mailed threats were received last month.

Last week, police obtained subscriber information for the e-mail account used by the sender of the e-mail posters.

The investigation is ongoing, but a spokeswoman for Zucchet told the Union-Tribune that "the police seem to be making some headway."

A federal grand jury indicted the councilmen Aug. 28, charging them with scheming to loosen the city's strip-club regulations in exchange for money and favors from strip club owner Michael Galardi and his associates. The councilmen have pleaded not guilty.