The ambiguity remains high after the act on Sunday night in the state of California.
Police announced on Monday that a 26-year-old man had been arrested, "with connections" to the shootings.
Task forces had then searched several houses, and found a handgun.
Several shot to death
At least six people, three men and three women, were shot dead and twelve injured 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.
Several deeds
Investigators describe the situation as "very complex and complicated".
"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.