Jones shot the third victim, Arnulfo Conrriguez, 28, in the neck at close range, police said. Conrriguez was in serious condition at Fresno's Community Regional Medical Center, hospital spokeswoman Mary Lisa Russell said.
A fourth victim, 32-year-old Fatima Lopez, witnessed the slayings and started to run. Jones shot at her, striking her in the buttocks. Lopez was treated at Community Regional Medical Center and released, Russell said.
Attempts by The Associated Press to reach Lopez for comment were not immediately successful.
Plant worker Estevan Catano, 21, escaped injury when the gun Jones placed against his head did not discharge because it was out of ammunition, police said. The gun only held four rounds.
Investigators secured Jones' Fresno apartment, where they found 24 rounds of .357-caliber ammunition — the type used in the shooting — and 21 rounds of .38-caliber ammunition.
Police said the handgun used by Jones — a .357 Derringer — is an expensive and rare weapon and they are trying to determine whether it was stolen. The serial numbers on the gun were filed off, Dyer said.
Valley Protein, where Jones worked for more than a year, was established in 2005, according to online business records. A call to the company went to a voicemail recording that said, "due to an emergency, we are closed for the day."
A woman who answered the phone at a listing for the company's CEO, Durbin Breckenridge, and identified herself as his wife said she would pass a message to him.
News media and onlookers were kept several blocks from the plant in the morning, as police used yellow tape to block access. Dozens of officers swarmed the area.
Joe Martinez, 45, told the Fresno Bee that he was in the drive-thru lane of a fast-food restaurant when he heard a loud pop that he initially thought was a car backfiring.
Then he looked to the north and saw a man on the ground with two people standing over him.
"It's the last thing you expect to see," the newspaper quoted Martinez as saying. "It's very upsetting."
Associated Press writers Jason Dearen, Terence Chea and John S. Marshall in San Francisco contributed to this report.