Police said they were reviewing a manifesto allegedly posted by a suspect in the shooting of a California synagogue Saturday, as they worked to establish a motive for the deadly attack.

San Diego County Sheriff Bill Gore told a press conference that investigators were reviewing the social media activity of a suspect detained after the shooting and establishing the legitimacy of an open letter published online.

Gore named the suspect as John Earnest and said he did not have a police record.

Get The Times of Israel's Daily Edition by email and never miss our top stories Free Sign Up

“We’re looking into digital evidence and checking the authenticity of an online manifesto,” the Sheriff’s Office said in a tweet.

Police said the 19-year-old man from the city of San Diego is believed to have entered a Chabad synagogue in Poway, California, during services on the last day of Passover, opening fire with an assault weapon and killing one person. Three others, including a girl, were wounded and are in stable condition.

Earnest surrendered to police after leaving the synagogue and calling to report the shooting, according to authorities.

Gore has declined to say what the motive for the crime was, but Poway Mayor Steve Vaus and others have said the shooting appears to be a hate crime.

A manifesto thought to have been posted online by the shooter is rife with anti-Semitic conspiracy theories and white nationalist rhetoric.

In it, Earnest cites Brenton Tarrant, who killed 50 people in attacks on two mosques in Christchurch, New Zealand last month, as an inspiration, as well as Robert Bowers, who killed 11 people in an attack on a Pittsburgh synagogue on October 27, exactly six months ago.

The manifesto is in the same question and answer format as one posted by Tarrant.

Earnest also takes responsibility for an arson attack on a Escondido, California, mosque immediately after the Christchurch attack and indicates he began planning the Chabad shooting shortly after.

He says he does not support US President Donald Trump, whom he calls a Zionist, and repeatedly accuses Jews of being part of a global conspiracy to put down white Christians.

The letter was posted on the 8chan messaging board as well as on a document hosting site at about 9 a.m. Pacific Time, about two hours before the shooting at the Chabad center.

Shortly after it was posted, users on 8chan cheered on the writer and encouraged him to get “a high score,” according to NBC News.

Speaking to reporters outside the White House, US President Donald Trump said investigators “we’re doing some very heavy research,” and that “it looks like a hate crime.”

Agencies contributed to this report.