Ten people were shot, four fatally, during a family's backyard gathering in Fresno, California, on Sunday (US time).
Fresno police Lt. Bill Dooley says the "mass casualty shooting" took place about 6pm Sunday on the city's southeast side.
Dooley says there was a party in the backyard of a home to watch a football game when the shooting suspects sneaked in and fired into the crowd.
Authorities have not released a total for the number of people killed.
"Officers arrived on scene – what we found were several individuals deceased in the backyard," Dooley told local media outlets at the scene on the 5300 block of East Lamona Avenue.
"What we do know is that this was a gathering, a family and friend gathering in the backyard.
"Everyone was watching football this evening when unknown suspects approached the residence, snuck into the backyard and opened fire."
The Bee reported that neighbours, who declined to be named for safety reasons, said they heard multiple gunshots.
Others were taken to Community Regional Medical Centre in critical condition, and some are in critical but stable condition, the TV stations reported.
No suspect is in custody.
Agents from the US Bureau of Alcohol, Tobacco, Firearms and Explosives field office in San Francisco are responding to the shooting, the Bee reported.
It was at least the second fatal gun attack Sunday in southeast Fresno, the Bee reported.
A man in his 20s was shot to death early Sunday at a home in another part of the city.
Police have not said whether the incidents could be connected