SACRAMENTO — The California Senate has confirmed that the red liquid an anti-vaccine protester threw on lawmakers last month was human blood, contained in a menstrual cup.

“While lab tests confirmed that the substance thrown from the Senate Gallery was human blood, it was negative for any blood borne pathogens or infections,” Secretary of the Senate Erika Contreras wrote in a letter Wednesday to staffers.

The letter, which The Chronicle obtained, also states that the Senate hired an environmental hazard company to “complete remediation and decontamination of both the chamber and the gallery,” which are now open to staff and the public.

Senators evacuated their chamber Sept. 13, on the final night of its session after an opponent of mandatory childhood vaccinations hurled a cup containing a red liquid at several lawmakers from the gallery above.

“That’s for the dead babies,” the woman shouted.

The California Highway Patrol said it arrested a woman who “threw a feminine hygiene device containing what appeared to be blood” at the lawmakers.

At least six senators were hit by either the menstrual cup or the blood it contained, and red splatters could be seen on some of their desks.

A video posted on Periscope showed a woman dressed all in black, repeatedly saying dead babies’ “blood is on your hands” as she was surrounded in a Capitol hallway by CHP officers. She was eventually handcuffed and led away.

The CHP identified her as Rebecca Dalelio, 43. According to online property records, she lives in Boulder Creek, outside Santa Cruz. She was booked on suspicion of six crimes, including assault and vandalism.

Dalelio has regularly shared posts on her Facebook page that support consipracy theories about vaccines. She was later released on bail, and her next court hearing is set for Nov. 18.

Dalelio could not be reached for comment Wednesday. Her father, Henry Dalelio, told The Chronicle he wasn’t surprised to hear his daughter had been arrested, saying she has “always been somewhat outspoken.”

“That’s just Becky being Becky,” he said. “Anybody who’s seen her on Facebook knows she’s opposed to vaccines.”

Henry Dalelio said his daughter didn’t get her “views from home, though,” adding that he and his wife “had all of the kids vaccinated. We kept them all up on their shots.”

Dustin Gardiner is a San Francisco Chronicle staff writer. Email: dustin.gardiner@sfchronicle.com Twitter: @dustingardiner