By Laura Zuckerman

SALMON, Idaho (Reuters) - Federal and state prosecutors in Idaho on Friday said they would not file criminal charges against two sheriff's deputies who fatally shot a rancher in November during a dispute over his bull, citing conflicting witness testimony and a spent shell casing in the man's rifle.  

The U.S. attorney's office in Boise and Idaho's attorney general said they could not prove beyond reasonable doubt that excessive or unconstitutional force had been used against rancher Jack Yantis during the incident on Nov. 1, 2015, near the farming community of Council in western Idaho.

They cited the shell casing that showed Yantis, 62, fired his rifle as a major reason for not pursuing charges in the confrontation that ended with Adams County deputies Cody Roland and Brian Wood firing 20 shots and hitting Yantis 12 times in the chest, upper extremities and abdomen, officials said.

The investigation found the shell casing in the chamber of Yantis' .204 bolt-action rifle, Idaho Attorney General Lawrence Wasden said in a letter to local prosecutors.

"This is likely the most critical piece of physical evidence since it indicates that (Yantis) actually fired his weapon and, at least to some extent, corroborates the officers' stories,” he said.

The family of the late rancher has accused police of excessive force in the case. Their attorney could not immediately be reached for comment.

However, Wasden also said conflicting statements by the deputies and Yantis' wife and nephew created enough doubt to not charge the deputies.

No audio or video recording of the shooting exists, the U.S. attorney's office said. Both deputies were wearing body cameras, but neither recorded the shooting as Wood's camera memory was full and Roland's camera was not activated.

U.S. prosecutors agreed in a statement that discrepancies in witness statements and the fact that forensic evidence showed Yantis fired his rifle made it unlikely they could prove the deputies should be charged.

The deputies responded to a report of motorists injured after their car collided with a bull near the Yantis ranch, Idaho State Police said. Wood unsuccessfully sought to kill the injured animal before Yantis, his wife and nephew arrived.

A disagreement between the deputies and Yantis occurred about whether his line of fire would injure bystanders, police said. The wife and nephew said one of the deputies got into a scuffle with Yantis and both officers fired their guns, while the deputies said Yantis aimed his rifle toward Roland and fired.

(Reporting by Laura Zuckerman; Editing by Ben Klayman and Bill Rigby)