Everett Basham, 45, was being held without bail in Santa Clara County Jail on suspicion of threatening a public official and possessing a concealed firearm. It was not clear if he had retained an attorney.
Adam Keigwin, a spokesman for Democratic Sen. Leland Yee of San Francisco, said Yee was the senator who received the threat and it was passed on to the California Highway Patrol. He said Yee isn't commenting at the request of the CHP.
"They need more time to investigate and we want to give them more time," Keigwin said.
But Keigwin said in an email late Wednesday to The Associated Press the threat to Yee was made in response to his legislative efforts to end gun violence.
Yee's SB47 would prohibit the use of so-called bullet buttons and other devices that allow for swift reloading of military-style assault weapons
Investigators found a loaded handgun in Basham's vehicle and explosive materials at his home, said Officer Sean Kennedy, a spokesman for the CHP's Protective Services Division.
The Santa Clara County sheriff's bomb squad blew up the materials at Basham's home after Basham's arrest Tuesday.
Neither Kennedy nor Keigwin would detail the threat, except to say it stood out as serious.
"All legislators get hate mail all the time, but we don't write search warrants," Kennedy said.
Basham "made some sort of threat that was serious or viable enough that the officers decided to go further," Kennedy said.
Yee has been the subject of frequent verbal and written threats, some of them seemingly racially motivated. Some came in January 2011 after he criticized talk radio host Rush Limbaugh's imitation of the Chinese language.
Later that year, officials who guard state Assembly members cited the threats against Yee and other lawmakers for their decision to begin carrying handguns full time. The shooting of U.S. Rep. Gabrielle Giffords and others in Arizona that year also prompted the decision to arm Assembly sergeants-at-arms.
Senate sergeants do not carry firearms.
No other possible targets have been identified in the most recent case, Kennedy said.
Basham worked briefly for Apple Inc. co-founder Steve Wozniak more than a decade ago, San Francisco-area media reported. Wozniak and his representatives did not return telephone and email messages from The Associated Press.
"I would classify Everett as distrustful of the government, as many of us are. He was a champion for the common person. Everett never spoke as a person who could or would commit a crime," Wozniak wrote in an email to NBC Bay Area, noting that he had attended Basham's wedding.
He told the San Francisco Chronicle that Basham "is not the sort of person who's a criminal or terrorist. He's just very brilliant."
Basham was arrested without incident at a relative's home.
Kennedy said a CHP special weapons team then entered Basham's home in a quiet neighborhood but backed out quickly after finding volatile chemicals and other apparent bomb-making materials.
"We weren't expecting to find the chemicals," Kennedy said. "That may have laid validation to the fact that this person may have had the intention to do harm."
The roof of the home is covered with antennas, satellite dishes and security cameras, and neighbors said they have seen Basham regularly wearing camouflage clothing. The interior of the home was so cluttered that investigators had difficulty opening the front door.
"We're just trying to get through it safely," Kennedy said. "After finding the chemicals, it makes you really nervous."
The chemicals were deemed too dangerous to be moved. The bomb squad burned the material without incident in a hole dug in the front yard.
"You've seen the movies. Some of the stuff, all it takes is a flick to set it off," Kennedy said.
Nearby neighbors were evacuated and others were told to stay in their homes while the materials were burned.