A correctional officer at the R.J. Donovan state prison in Otay Mesa is recovering from a graze wound after being hit in the head by a stray bullet Thursday.
The round came from a nearby firing range, where a still-unidentified local law enforcement agency was training.
The outdoor range is located about 300 yards east of the main prison compound, where the officer was stationed near the perimeter fencing.
At about 8:30 a.m., he was struck in the back of the head, and wound up bleeding but still conscious.
"He believed that somebody had actually hit him in the head with a rock or something of that sort," said Lt. Patrick Logan, spokesman for the 4,600-inmate correctional facility.
"There was no entry or exit wound," Logan added. "(The officer) felt the area on his head where he felt the impact. He noticed some bleeding, and of course we summoned medical assistance immediately."
The officer was taken to Scripps Mercy Hospital in Hillcrest, where he was treated for a graze wound, and released before noon to recuperate at home.
Investigators at first proceeded on the theory that the stray bullet was a ricochet from the firing range, where there was small-arms, close-quarters training in simulated room-entry scenarios, involving partitions.
But they say it's possible there was a misfire, with the round -- apparently from a semi-automatic pistol -- taking an upward trajectory over a steep dirt berm toward the west, and then falling into the prison compound.
Several agencies are now looking into the accident.
"We're not really sure what the extent of it is -- if it was a ricochet, if it was a misfire," said Logan. "We have no idea at this point."
Prison authorities say this was the first gunfire-related injury involving the firing range since R.J. Donovan opened nearly 25 years ago.