Speaking to MSNBC, Steve Vaus, the mayor of Poway, said there had been "one fatality" and called the "horrific" attack a "hate crime".
The author, who introduces himself as a "man of European ancestry" and "a descendent of one of the original colonists", says that although he as an "individual" may kill only a small number of Jews, he hopes that his grim act would inspire others "to take a stand".
Earlier, a source told CBS News that a rabbi was among the people who were shot. It said that there were injuries but that no additional details were immediately available. About 100 people were inside at the time, according to Gore.
"This is not Poway", Vaus told reporters.
Walter Vandivort, who lives in the neighborhood of the synagogue, said he heard gunshots while he was indoors.
The San Diego Police Department and Federal Bureau of Investigation are working in combination with the U.S. Attorney General's Office to investigate.
Authorities said an off-duty Border Patrol agent fired at the shooter as he was fleeing the scene, but didn't hit the suspect.
Elderly man diagnosed with measles in Pinellas County
In 2017, some 110,000 people, a lot of them children, died from measles - a 22 percent increase from the year before, UNICEF said. According to Unicef estimations, 169 million children worldwide missed the first dose of the measles jab between 2010 and 2017.
Passover began on April 19 and was ending Saturday.
A large group of congregants had gathered behind the temple after the shooting, sheriff's Sgt. Aaron Meleen said.
According to Gore, authorities have copies of the suspect's social media accounts and an open letter, which they have yet to release.
The post said he was training to be a nurse and cited the suspect accused of carrying out the deadly attacks on mosques in Christchurch last month, and at Pittsburgh's Tree of Life synagogue on October 27 where 11 people were killed. The witness said the rabbi tried to calm the gunman and members of the congregation even after the rabbi was wounded, the station reported. The Chabad of Poway announced on Facebook on Thursday that it would be holding a celebration at 11 a.m. on Saturday, and a final Passover meal at 7 p.m.
He said: "As you can imagine it was an extremely chaotic scene with people running everywhere when we got here".
The president first tweeted, "Thoughts and prayers to all of those affected by the shooting at the Synagogue in Poway, California".
Congregation Chabad, where the incident took place is located at 16934 Chabad Way.
John Earnest, the 19-year-old man arrested in connection with today's Chabad of Poway Synagogue, reportedly published a manifesto prior to the attack.