The swans were found dead in a lake in Zhenglan Banner where migrating swans had stopped in late October.
Tests showed that they were poisoned with carbofuran, a type of pesticide, which was detected on the dead birds, state-run Xinhua news agency reported.
Police spent nine days searching for the suspects and caught them in Beijing and Tianjin municipalities as well as the provinces of Jilin and Hebei as of yesterday, according to the information office of Zhenglan Banner in Xilingol League.