A series of anti-Muslim attacks in recent weeks have referenced the murder of 50 Muslims at a Christchurch, New Zealand mosque earlier this month.

The attack on Christchurch’s Al Noor mosque on March 15 saw a white supremacist open fire on worshippers in a massacre designed to go viral. Shortly before the attack, the alleged terrorist promoted a manifesto and a Facebook link where people could watch the shooting in real time. Since then, a rash of alleged anti-Muslim attacks in the U.S. and U.K. have referenced the attack.

Seven people were inside Escondido, California’s Dar-ul-Arqam mosque early Sunday morning when the outside of the building erupted in flames. Worshippers extinguished the fire before it could hurt anyone, but the outside of the mosque sustained minor damage. Law enforcement determined the blaze to be an arson, aided by an accelerant.

The arsonist left a message. “Graffiti left behind by the suspect made reference to the shooting incident in New Zealand,” Escondido police said in a statement. The fire is currently under investigation as a hate crime.

Lack of hate crime data in the U.S. means it's too early to tell whether anti-Muslim attacks are on the rise after the Christchurch shooting, said Madihha Ahussain, special counsel for anti-muslim bigotry with the group Muslim Advocates. But the attack has sparked safety concerns among American Muslims.

"What we have noticed is that mosques and institutions are expressing greater concerns about security," Ahussain told The Daily Beast. A high-profile attack "triggers these types of opportunities for bigots to express their views and to use this as an avenue and an opening to continue to create fear and an environment of hatred and bigotry for these communities."

Meanwhile, watchdogs in the U.K. have reported a surge in anti-Muslim attacks in the aftermath of the Christchurch shooting. Tell Mama, a U.K. organization tracking anti-Muslim incidents recorded 95 anti-Muslim attacks and online threats between the March 15 Christchurch attack and March 21. Eighty-five of those attacks “contained direct references to the New Zealand attacks,” the Guardian reported. Those figures include online threats. Of the 85 references to the Christchurch attack, 40 incidents were online and 45 occurred in person.

Tell Mama said they usually register about 30 to 35 anti-Muslim incidents each week. British officials are currently investigating hammer attacks on five Birmingham mosques in the days after the New Zealand attack. Security footage appears to show a man approaching mosques before dawn and swinging a heavy object at their windows.

Hate crimes trends are harder to track in the U.S., said Heidi Beirich, head of the Southern Poverty Law Center’s Intelligence Project.

“Our data on hate crimes is always outdated,” Beirich told The Daily Beast of data collected by U.S. authorities. “We just got the [FBI’s] 2017 hate crime data, and we think those crimes are always severely undercounted.”

The FBI’s 2017 hate crime report found a 17 percent rise in hate crimes that year. But experts caution that the numbers are imprecise. The FBI collects it data from local law enforcement agencies, which are not required to track or turn over hate crime statistics. Those that do might have classify the crimes differently, leading to an incomplete picture of hate in America.

After a white supremacist allegedly murdered 11 worshippers in a Pittsburgh synagogue last year, college campuses around the country reported a surge in anti-Semitic graffiti and vandalism, although the correlation between the Pittsburgh attack and the new incidents of anti-Semitism is hard to quantify without thorough data.

But where hate crime data takes more than a year to emerge, officials worry about more immediate patterns of bigoted attacks, Beirich said.

“If you listen to the behavioral scientists at the FBI, they’re constantly worried about copycat attacks, or the sensationalization of these attacks,” she said.