Violent attacks such as Sunday’s shooting at the home of U.S. District Judge Esther Salas are impossible to prevent, according to judicial security experts, but the likelihood of such incidents can be mitigated.

Foiling attacks against judges is often a challenge because individuals who commit such crimes rarely signal their intentions. But vigilance about certain types of behavior and open communication with the U.S. Marshals Service can reduce the risk, according to some security experts.