Helaman 1:12

therefore Kishkumen was not known among the people of Nephi, for he was in disguise at the time that he murdered Pahoran. And Kishkumen, and his band which had covenanted with him, did mingle themselves among the people, in a manner that they all could not be found; but as many as were found, were condemned unto death.

So the Nephites did not know who Kishkumen was. He wore a disguise when he murdered Pahoran. Kishkumen and his group mixed in with the people so they could not all be found. But those who were found were sentenced to death.

Nobody knew who Kishkumen was because he’d worn a disguise. He and his whole gang blended back into the crowd. The authorities hunted them down, executing anyone they caught, but most got away.