URUMQI, China  As the desert region of Xinjiang in northwest China settled into tense stillness after three days of deadly ethnic violence, a Communist Party leader from the region said that those directly responsible for the killings of 156 people in the initial rioting on Sunday would be punished with the death penalty.

The official, Li Zhi, the head of the party in Urumqi, which is Xinjiang’s capital and the center of the violence, said that many people suspected of being instigators had been arrested and that some were students.

“To those who have committed crimes with cruel means, we will execute them,” Mr. Li said at a morning news conference. “The small groups of the violent people have already been caught by the police. The situation is now under control.”