Republican presidential candidate Donald Trump poses for a selfie with an audience member after Trump called him up onstage at a campaign rally in Claremont, New Hampshire, Jan. 5, 2016. Reuters

Republican presidential candidate Donald Trump is citing a spate of recent crimes in Germany on New Year’s to bolster one of his central campaign messages: Immigrants are bad.

“Germany is going through massive attacks to its people by the migrants allowed to enter the country. “THINK!” the GOP front-runner wrote on Twitter Wednesday, referring to a series of sexual assaults reportedly conducted by North African- or Arabic-looking men over the holiday.

On New Year’s Eve, as many as 1,000 young men circled women in the German city of Cologne groping and, in one reported case, raping women. German police have said, however, that there is no clear indicator that the attackers arrived over the past year from conflicts in places like Syria, Iraq or Afghanistan. No arrests have been made so far.

Trump’s implied opinion shows just how much of a tinderbox anything related to migrants has become in the aftermath of the Paris terrorist attacks in November. Those attacks, which have been linked to the Islamic State group, resulted in heightened concern in Europe and the United States that the massive, ongoing influx of asylum-seekers and migrants from the Middle East and North Africa could pose a substantial threat to the security of Western nations. Still, while other European countries began considering new immigration policies, German Chancellor Angela Merkel has been steadfast in her country’s commitment to help arriving asylum-seekers. Last week’s crime sprees could pose a political challenge to her going forward, and some German politicians have already taken somewhat drastic positions.

“There’s always the possibility of keeping a certain distance of more than an arm’s length – that is to say to make sure yourself you don’t look to be too close to people who are not known to you, and to whom you don’t have a trusting relationship,” Henriette Reker, Cologne’s mayor, said this week when asked how women can protect themselves from attacks like those on New Year’s Eve.

Trump has been a leading conservative voice in the U.S. when it comes to immigration. He began his presidential campaign in June by denouncing Mexican and Central American immigrants, saying that Mexicans are rapists and criminals. He has since taken a hard stance on other groups, including Muslims, whom he has said should be barred from entering the United States until Congress has the chance to understand the threat the individuals may pose.