On 21:9 Rashi (quoting Gemoro Sotah 47) says that if the murderer is found after the decapitation of the heifer then he must be executed. How (asks Chizkuni) can he be executed without witnesses?
Who says Rashi is talking without witnesses? He means if you have witnesses then he is killed.
The Targum Yonason says that worms crawled from the deceased’s body until they found the murderer. Since a miracle happened maybe witnesses are unnecessary. Couldn’t find the Chizkuni?