At least six people, three men and three women, were shot dead and 12 injured in Sacramento when several shooters opened fire as people streamed out of bars and clubs that closed for the night.
At least four of the injured are being treated with life-threatening injuries, according to the authorities.
On Sunday, extensive work was underway to fine-tune and map the crime scene, which stretched for several blocks in the central parts of the city.
The investigators try to outline the course of events surrounding the shooting and describe the situation as "very complex and complicated".
"Once again, the United States is in mourning"
The suspects are still at large, but police have found a stolen gun and are reviewing videos on social media that were taken in connection with the shooting.
"Once again, the United States is in mourning for another society that has been devastated by gun violence," said US President Joe Biden in a statement.
The shooting was the third this year in the United States in which at least six people have been shot dead at one and the same time, according to a database compiled by the news agency AP, the newspaper USA Today and Northeastern University in Boston.