Three of the letters found earlier caught fire, but no one has been injured.
The letter to Gov. Arnold Schwarzenegger was spotted Monday by screeners who routinely inspect letters intended for the governor and other members of the state's executive branch at a California Highway Patrol center in West Sacramento, CHP spokesman Tom Marshall said.
Schwarzenegger's office had no comment Tuesday.
New Mexico Gov. Bill Richardson confirmed Tuesday that he also had been sent one of the rigged letters.
Brian Grace, director of mail operations for the California governor's office, said the thick, heavy envelope he spotted Tuesday morning matched the description of letters sent to other governors, including a return address from Nevada's maximum-security Ely State Prison. It was postmarked Sept. 6.
"I called the security staff … so they took it away," Grace said.
Authorities have said they were interviewing correctional officers and inmates at the Nevada prison and have narrowed their investigation to "a person of interest."
Marshall said authorities initially decided "because of the security issue" not to announce the interception. But the CHP confirmed the letter Tuesday after receiving numerous calls asking about bombs and terrorist materials allegedly intended for the governor.
Other governors targeted so far include those in Alaska, Arizona, Colorado, Hawaii, Idaho, Massachusetts, Montana, Nebraska, Nevada, New York, Oregon, Texas, Utah, Vermont, Virginia, Washington, West Virginia and Wyoming.
In addition to the 13 Republican and seven Democratic governors, Nevada's corrections director received a booby-trapped letter.
Federal authorities are investigating the mailings. U.S. Postal officials said sending a letter intended to harm the recipient is a federal felony, with a possible prison sentence up to 20 years.
Some well-known U.S. crimes have been committed through or involving the postal system, including: