GOLDEN — The Arapahoe County sheriff has opened an internal investigation into the actions of an off-duty deputy who shot and killed a man in a traffic altercation.

The probe of Deputy Daniel Montana Jr. in the Nov. 2 shooting was launched Monday — the same day it was announced a Jefferson County grand jury did not indict the deputy.

“I didn’t think it was appropriate to conduct an internal investigation when there’s grand jury proceedings,” Arapahoe County Sheriff Grayson Robinson said.

Robinson said he didn’t know how long the investigation would take. Montana, who remains on administrative leave, has been assigned to the detentions unit since he joined Arapahoe County in 1991.

A grand jury decided late Friday not to indict the deputy. The family of David Rossiter, 25, of Sheridan, Mont., was told Sunday of the grand jury’s decision by Jefferson County District Attorney Scott Storey, who traveled to Montana to deliver the news in person.

“They are shocked and disappointed,” said the Rossiter family’s attorney, Stephanie Kruer. “The Rossiters put their faith in the criminal-justice system, and they are astounded the grand jury came to the decision they did.”

“I felt (a) personal sense of obligation” to talk with them, Storey said. He described the private conversation as “difficult, very difficult.”

“It’s a wonderful family, very dignified, very polite,” Storey said.

Kruer said the family has directed her to file a civil case, which should be done within the next few weeks.

Lakewood police said Montana, 49, of Golden, shot Rossiter during a roadside confrontation at West Sixth Avenue and Indiana Street.

The altercation involving Rossiter and Michael Hunter, the driver of the pickup in which Rossiter was riding, may have begun with a cigarette butt tossed from the truck that landed on Montana’s vehicle.

The autopsy report released Monday showed the fatal bullet broke three ribs before piercing Rossiter’s right lung and heart. The “large-caliber, copper-jacketed lead bullet” was recovered during the autopsy.

Rossiter had cuts and a broken middle finger and thumb on his right hand, according to the report, and the knuckles on his left hand had cuts, scrapes and bruises.

A second bullet entered Rossiter’s thigh and traveled upwards, exiting through Rossiter’s buttock. That bullet was not recovered.

Criminal charges are unlikely to be filed unless there is new or additional information.

Without commenting about possible charges, Storey said, “The grand jury’s burden of proof is probable cause.”

A prosecutor’s burden, which is more demanding, is to prove, beyond a reasonable doubt, that a crime has been committed, he said.

Storey said he decided to present the case to a grand jury because grand juries “are a good barometer of the community standard, especially in a case like this where there is loss of life.”

The grand jury process “aided this investigation,” Storey said. “This investigation was very thorough and meticulous.”

Storey said that because of secrecy requirements in state law, he could not comment on what was presented to the grand jury, nor could he comment on why the grand jury did not issue an indictment.

Ann Schrader: 303-278-3217 or aschrader@denverpost.com

A clarification ran on this article, in print. This version reflects more detailed information about the autopsy findings,