The shooting was reported shortly before 1:30 p.m. at Geneva Presbyterian Church in the town of Laguna Woods, the Orange County Sheriff’s Department said on Twitter.
A man died at the scene and a fifth injured person suffered minor injuries, officials said. All of the victims were adults.
Deputies arrested a suspect, an adult male, and recovered a weapon from the scene, officials said. It was not immediately clear where inside the church the shooting took place.
About 30 people witnessed the violence, said Carrie Braun, a sheriff’s spokeswoman. The majority of people inside the church are believed to be of Taiwanese descent, Braun said.
Investigators considered many factors, including whether the bloodshed could be a hate crime and whether the shooter was known to the church community, she said.
More details were expected from a sheriff’s department press conference scheduled for 5 p.m.
Federal agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives were responding to the scene. The FBI was also sending agents to the scene to help the sheriff.
Laguna Woods was built as a senior citizens’ community and later became a town. More than 80% of residents in the city of 18,000 about 80 miles southeast of Los Angeles are at least 65 years old.
Governor Gavin Newsom’s office said on Twitter that it was monitoring the situation closely.
“No one should be afraid to go to their place of worship. Our hearts go out to the victims, the community and everyone affected by this tragic event,” the tweet read.
The incident happened in an area with a cluster of places of worship, including Catholic, Lutheran and Methodist churches and a Jewish synagogue.
On its website, the Presbyterian Church of Geneva describes its mission as “remembering, telling and living the way of Jesus by being just, kind and humble”.
“All are welcome here. Truly, we mean that! … Geneva aspires to be an inclusive congregation of worshiping, learning, connecting, giving and serving together.
The shooting came a day after an 18-year-old man shot and killed 10 people at a supermarket in Buffalo, New York.
“This is shocking and disturbing news, especially less than a day after a mass shooting in Buffalo,” said U.S. Representative Katie Porter, whose district includes Laguna Woods. “This shouldn’t be our new normal. I will work hard to support the victims and their families.