"slain" (connoting: by the sword), and not strangled. "slain": and not convulsing.
"which the L-rd your G-d gives to you": to include (the land of the Jews) across the Jordan.
R. Eliezer says: In all (of these instances) if he were slain (by the sword), they would break the neck (of the heifer). R. Yossi b. R. Yehudah said to R. Eliezer: If he were strangled or cast in the refuse, would they break the neck? So, in all of these instances, they would not break the neck.
"it was not known who killed him": But if it were known who killed him — even if one person in the end of the world had slain him, (though normally two witnesses are required) they would not break the neck.