U.S. Court of Appeals for the Ninth Circuit Judge Ryan Nelson recused himself Thursday from hearing a case in which Palestine human rights groups claim U.S. military aid to Israel’s war efforts in Gaza has violated international laws barring genocide.

The groups said Nelson’s impartiality could be questioned based on a trip he took to Israel in March with 13 other federal judges that was sponsored by the World Jewish Congress. The trip, in which judges met with Israeli government and judicial officials, appeared to be designed to influence U.S. judicial opinion on the legality of Israeli military action in Gaza, the groups said in a recusal motion.