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Terrorist attacks often fuel online hate and increase the expression of xenophobic and anti-minority messages. Previous research has focused on the impact of terrorist attacks on prejudiced attitudes towards groups linked to the perpetrators as the cause of this increase. We argue that social norms can contain the expression of prejudice after the attacks. We report the results of a combination of a natural and a lab-in-the-field experiment in which we exploit data collected about the occurrence of two consecutive Islamist terrorist attacks in Germany, the Würzburg and Ansbach attacks, in July 2016. The experiment compares the effect of the terrorist attacks in hate speech towards refugees in contexts where a descriptive norm against the use of hate speech is evidently in place to contexts in which the norm is ambiguous because participants observe anti-minority comments. Hate towards refugees, but not towards other minority groups, increased as a result of the attacks only in the absence of a strong norm. These results imply that attitudinal changes due to terrorist attacks are more likely to be voiced if norms erode.