The red liquid that was thrown onto the California state senate floor by an anti-vax protester, just as lawmakers were set to roll back vaccine exemptions in September, was in fact human blood, tests have confirmed.

"That's for the dead babies," Rebecca Lee Dalelio yelled on Sept. 13 after she tossed what reportedly looked like a menstrual cup with red liquid onto the floor.

In a letter to staffers on Wednesday, the secretary of the senate, Erika Contreras, said lab tests confirmed the blood was human, but "it was negative for any blood borne pathogens or infections."

The letter, obtained by BuzzFeed News, did not state whether the blood was menstrual in nature. Some of the blood did land on lawmakers, including Steve Glazer, who underwent a medical checkup the following day.

The attack, on the last day of the state senate's session, was the latest escalation from anti-vax groups who descended on Sacramento as legislators considered the strictest vaccine law in the country.