The FBI issued a statement on Sunday warning of people trying to imitate the recent attacks in El Paso, Texas, and Dayton, Ohio.

“The FBI remains concerned that U.S.-based domestic violent extremists could become inspired by these and previous high-profile attacks to engage in similar acts of violence,” the statement said. “The FBI asks the American public to report to law enforcement any suspicious activity that is observed either in person or online.”

The FBI’s Domestic Terrorism-Hate Crimes Fusion Cell has joined the investigation into the August 3 El Paso shooting that killed 22 innocents, as well as the Dayton shooting that slew 9. They fear that the perpetrators, like Patrick Wood Crusius, might “inspire” others to take similar violent action.

FBI Director Christopher Wray expressed “sincere condolences to the victims, families, and communities affected by this weekend’s violence” in a statement issued on Sunday.

“I am proud of our state and local law enforcement partners and the immediate response of FBI agents, analysts, and professional staff, working in close coordination to assist them,” he said. “I have been in contact with the president and the attorney general, and they both have expressed their support for the FBI’s work in the wake of these tragedies.”

Copycat crime is far from a new idea. While it has been observed and theorized since the 1800s, research remains sparse at best. And in a modern world soaked with information from a 24/7 news cycle, the threat of deadly imitation is more relevant than ever.