It's the latest event in what has been a tumultuous year for the dozen or so officers of this small, independently-run police agency.
School district officials were notified of the investigation Friday. "We just received the papers today notifying us that there is a grand jury investigation" into the police force, Oakland Unified School District spokesman Troy Flint said.
Last August, Sarna resigned following charges that he hurled racist remarks at one of his African-American officers after a charity golf outing.
Attorney Joe O'Sullivan represents another fellow officer who witnessed that racially charged incident and reported it to school officials.
"Yesterday the Federal Bureau of Investigation notified my client that they were initiating a formal criminal investigation of the Oakland Unified School District's police department," O'Sullivan said.
He said since his client blew the whistle on Sarna, he's been ostracized by his fellow officers.
"Relative to my client, I think it's been quite discriminatory. And also towards the African-American victim of the racial epithets. They've not been assimilated back. There's been retaliatory measures made."
Flint said he wasn't aware of any allegations or potential threats of retaliation. He said, "There was some turmoil. Obviously, when any unit loses its head, it takes some time for everything to restabilize.
Sarna became the school police chief following his resignation from the California Justice Department in 2007. He left that post after he crashed a state-issued car and was cited for driving while intoxicated.
After his resignation from the Oakland School Police, community activists voiced concern when school sergeant Bharin Bhatt was appointed interim chief.
School officials say Bhatt killed Raheim Brown when he shot the 20-year-old several times after he threatened the officer with a screwdriver.
Bhatt was replaced with James Williams, who is currently serving as chief.
"We're very happy with his leadership," said Flint. "Most likely the investigation is not dealing with events that occurred during his tenure."
Oakland Unified Shools says it will cooperate completely with the federal inquiry. But the school district does not yet know the full extent or scope of the investigation.
The U.S. Attorney's Office and the FBI in San Francisco both said they cannot comment on any pending investigations.