Bypassing waiting patients, a gunman who complained he had a botched 2010 surgery opened fire with a 12-gauge shotgun at doctors at a Reno medical office – a deadly rampage that investigators say was referred to in a suicide note.

On Thursday, detectives provided a chilling, detailed account of how Alan Oliver Frazier of Plumas County, California, opened fire with a pistol-grip shotgun in the examination room area of a urology medical practice.

The 51-year-old killed one doctor, critically wounded another and seriously injured a 20-year-old woman with a shotgun wound to the torso, before shooting himself to death.

"We may never know the suspect's exact motive or reason for his actions but there is indication that his focus was on the physicians at this specific office," said Reno police Lt William Rulla.

Police said they searched Frazier's home in northern California and found evidence that he was plotting the attack."Inside, we located a typed letter indicating the suspect's intention to commit this horrific act," Rulla said. "We also located other firearms within the residence, as well as notes indicating the suspect's actions during this incident were to be his final actions."

Witnesses told investigators Frazier also made statements at the time of the attack that he was looking for physicians, not patients, Rulla said.

Frazier entered the third-floor office about 2pm Tuesday with the shotgun he used to kill Dr Charles G Gholdoian, 46, president of Urology Nevada; critically wound Dr Christine Lajeunesse and seriously injure Shantae Spears, who was accompanying a relative on an office visit.

Frazier also was carrying a two-shot Derringer pistol and a fully loaded .40-caliber handgun, neither of which police believe he fired. He walked past the reception desk and toward the examination rooms where he first encountered Lajeunesse in the hallway and blasted her at point-blank range with one shot that hit both her arm and torso, police said.

As she fell to the floor, he continued down the hall and fired at Gholdoian, missing him but striking Spears in the torso. He fired again and hit the doctor with a fatal shot to the torso. Frazier fired a fourth shot into a smoked glass door, perhaps in an attempt to gain access, Rulla said, then "returned to the main lobby area where the fifth and final shot was the self-inflicted gunshot wound to his head."

Police said Frazier had a surgery in 2010 and claimed to have adverse symptoms because of it. But it was not immediately clear whether the unspecified procedure was performed at Urology Nevada or if either of the two doctors who were shot had been involved in the operation. Detectives were securing Frazier's medical records to determine the nature of his surgery and other details.

Plumas County sheriff Greg Hagwood said his agency helped Reno police execute a search warrant Wednesday at the home where Frazier lived alone near Lake Almanor, about 130 miles north of Reno. He last worked as power plant control room operator for Mt Lassen Power Co in a neighboring town, police said.

Rulla said they also recovered the pickup truck Frazier drove to the Renown Regional Medical Center campus on Tuesday and were in the process of serving a search warrant on the vehicle where it's being held at the police station. He said they've found no criminal history on him so far in either Nevada or California.

Frazier's former fiancee, Stephanie Wright-West, told the Associated Press on Thursday that Frazier took medication for depression when they were together in the mid-1990s.

Lajeunesse remained in critical condition and Spears in serious condition on Thursday at the medical center, Renown officials said. Local clergy were hosting an interfaith candlelight vigil Thursday night at a park across the street.