A wide array of factors contribute to the incidence of hate crimes. Ignorance and isolation may play a role; most Americans say they do not personally know any Muslims, although those who do report positive views of Muslims in general. The nature of the threat groups of people are perceived to pose can also be a factor; prejudice catalyzed by a terrorist attack, for example, may be particularly likely to inspire hate crimes. Political rhetoric is only one ingredient in that mix, and the many messages in circulation after an attack can make it harder to determine the impact of any one particular reaction from a political leader. Before Trump’s call for a ban on Muslims entering the country, President Obama delivered a speech to the nation on the San Bernardino attack stressing tolerance.

Still, the report looked at daily data following terrorist attacks, and found that “a tolerant statement about Muslims by a political leader was accompanied by a sharp decline in hate crime, while a less tolerant announcement was followed by a precipitous increase in both the severity and number of anti-Muslim hate crimes.” It notes that “there have been very few incidents of actual hate crime where Mr. Trump’s name was uttered since his candidacy,” but adds that “the increase of 87.5% in anti-Muslim hate crime in the days directly following his announcement is a troubling development and worthy of concern.”

Aside from calling for a ban on Muslims entering the the United States, Trump has said that “Islam hates us,” and accused American Muslims of protecting terrorists. The research does not demonstrate a direct causal link, nor can it rule out the role of other factors. It’s possible that the documented increase reflects an increase in hate-crime reporting due to heightened awareness of Islamophobia, which has become a topic of discussion during the presidential race. Nevertheless, the research does raise the possibility that Islamophobic political rhetoric may have devastating consequences.

A Georgetown University report released in May similarly found that threats, intimidation and violence against Muslim Americans have surged over the course of the presidential election. Engy Abdelkader, the author of the report, believes that trend is linked to Trump’s political rise. “Trump has seized on people’s fears and anxieties,” Abdelkader said. “I think that has translated in a number of instances not just to hostility, but acts of violence.”

Islamophobia existed in America long before Trump. Muslims have long been particularly vulnerable to backlash driven by negative stereotypes in part because they make up a relatively small slice of the overall U.S. population. But when a major-party nominee endorses and reinforces those stereotypes, researchers warn, American Muslims face serious risk of increased marginalization and outright violence.