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A letter mentioning "ultra sonic waves" was found after a gunman fatally shot a California police officer without warning this week before firing at others and later killing himself.

The letter was found face-up on his bed, said Lt. Paul Doroshov of the Davis Police Department. The Yolo County Sheriff's Office deputy coroner identified the suspect Saturday as Kevin Douglas Limbaugh, 48, of Davis, California.

Kevin Douglas Limbaugh Yolo County Sheriff's Dept.

Limbaugh is believed to have opened fire on Davis police Officer Natalie Corona, 22, without warning as she responded to a three-vehicle traffic collision that was reported around 6:45 p.m. Thursday, officials said. Limbaugh was not involved in the accident, and the police chief said it appeared to be "an ambush."

The typed letter, released by police Saturday, says, "The Davis Police department has been hitting me with ultra sonic waves meant to keep dogs from barking." He went on to say that he notified the press, internal affairs and the FBI about it.

"I am highly sensitive to its affect on my inner ear. I did my best to appease them, but they have continued for years and I can't live this way anymore," the letter reads. It is not signed, but typed at the bottom is "Citizen Kevin Limbaugh."

Natalie Corona is sworn in the Police Department by her father, Merced, on Aug. 2, 2018. Williams Pioneer Review

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After shooting Corona without warning, Limbaugh opened fire in another direction, including towards a firefighter who was running away, but no one else was injured, Davis police Chief Darren Pytel said Friday. A bullet hit the firefighter in the boot, but he was not hurt. A motive is under investigation.

Pytel said there were two firearms, which he said were semi-automatic pistols. On Nov. 9, Limbaugh relinquished a Bushmaster AR-15 rifle to Davis police following a prior conviction, according to court documents.

He was arrested and charged with battery with serious injury in Yolo County on Sept. 20, according to court documents. Online court records indicate that he pleaded no contest and was sentenced to probation. The Sacramento Bee newspaper, citing a source, reported Saturday that the battery case stemmed from Limbaugh punching a co-worker at Cache Creek Casino in the face after a dispute.

Doroshov did not immediately respond to an email seeking more details about the letter.

"When the investigators got into the house where the suspect was, they found a letter, open-face, on the bed," Doroshov told NBC affiliate KCRA of Sacramento. "We can only speculate that it was there to be seen immediately."

Christian Pascual, 25, who was involved in the fender-bender, told the Sacramento Bee in an article published Friday that the drivers were exchanging information with Corona when a bystander came up behind his right shoulder and opened fire.

Pytel said Friday that witnesses reported the gunman rode up on his bicycle and "was in some shadows on the sidewalk" as Corona was in the street between two vehicles, and "the suspect basically just opened up fire," shooting Corona once and that "he then ended up shooting her multiple times."

"At that point, he unloaded a magazine, so he expended all the rounds in the magazine, reloaded and then started shooting in another direction," Pytel said.

The gunman had dropped his backpack, and information inside led police to him, Pytel said.

Police later surrounded a house one block away from the shooting scene, where they found the suspect, who had killed himself, authorities said.

Pytel said police had prior contact with the gunman "but nothing that would indicate this type of issue."

Corona started working for the Davis Police Department in 2016 in community services, eventually going to the academy and graduating last July, officials said. She completed her training just before Christmas and became a full-time officer.

Pytel said Friday that Corona was "an absolute star in the department."

Davis is a city of 65,600, west of Sacramento, according to 2010 census data.