By Daniel Trotta

(Reuters) -A Jewish man who fell to the ground in an altercation amid pro-Israel and pro-Palestinian street protests in California died on Monday, and his death was determined to be a homicide, law enforcement and a Jewish organization said.

Paul Kessler, 69, died of blunt force head trauma a day after the altercation on Sunday, the Ventura County Sheriff's Office said, adding that witnesses described the incident as battery.

As of Monday night, no suspect was in custody in what the sheriff's office said "appears to be isolated and not part of a large effort," though it had not ruled out a hate crime.

The incident took place as emotions have run high in the United States over the war between Israel and the Palestinian militant group Hamas, with U.S. officials and civil rights groups warning of increased threats against Jews, Muslims and Arab Americans since fighting broke out on Oct. 7.

Last month an Illinois man was charged with hate crimes for stabbing a 6-year-old Muslim boy to death and wounding his mother in an attack that officials said targeted them for their religion in a response to the war.

The Jewish Federation of Greater Los Angeles identified the victim as a Jewish man and labeled the incident as the fourth act of antisemitic violence in the Los Angeles area this year and the second since Oct. 7.

Separate pro-Israel and pro-Palestinian demonstrations occurred simultaneously on Sunday in the city of Thousand Oaks, about 40 miles (65 km) west of Los Angeles, the sheriff's office said.

Kessler was involved in a physical altercation between counter-protesters, the sheriff's office said, citing witness accounts. It did not specify which side instigated the altercation.

"During the altercation, Kessler fell backwards and struck his head on the ground. Kessler was transported to an area hospital for advanced medical treatment. On November 6, 2023, Kessler succumbed to his injuries," the statement said.

Deputies asked the public for help in what it called "an active and ongoing investigation." The sheriff's office said it would hold a news conference on Tuesday about the incident.

The leader of the Jewish Federation of Greater Los Angeles, citing conversations with local government officials, said a pro-Palestinian protester had struck the victim on the head with a megaphone.

Rabbi Noah Farkas, president and chief executive of the group, said investigators have identified the person but that they have not made an arrest because the probe was ongoing.

"This is what's happening in American right now. There is a culture of fear and a culture of really terror against the Jewish community happening," Farkas said.

Kessler, who was carrying an Israeli flag at the demonstration, came from a family of philanthropists who were devastated by his death, Farkas said.

The Los Angeles chapter of the Council on American-Islamic Relations, a civil rights group, also expressed grief over what it called a "tragic and shocking loss," while also asking people to "refrain from jumping to conclusions" or "sensationalizing such a tragedy for political gains."

"CAIR-LA and the Muslim community stand with the Jewish community in rejecting any and all violence, antisemitism, Islamophobia, or incitement of hatred," the statement said.

(Reporting by Daniel Trotta; editing by Rami Ayyub, Robert Birsel and Gerry Doyle)