Police records have raised the possibility that there is a legitimate grievance behind Christopher Dorner's homicidal vendetta against theLos Angeles police department.

Dorner, a former LAPD officer, was sacked in 2008 for falsely claiming a colleague kicked a homeless suspect, but a judge who examined the case concluded that he could not be certain whether or not the suspect had been kicked.

The revelation came amid mounting frustration that a massive five-day manhunt had failed to catch Dorner, 33, who is accused of killing three people in a self-declared "war" against police and their families in southern California.

On Monday the Riverside county district attorney charged Dorner with murdering a police officer and the attempted murder of three other officers.

Police continued searching cabins in remote areas of Big Bear, a snowy mountain resort north of LA which has been the focus of the dragnet, and officers elsewhere chased up reported sightings. Authorities declined to confirm whether drones were being used.

On Sunday, LA mayor Antonio Villaraigosa announced a $1m reward for information leading to Dorner's capture, saying: "We will not tolerate this reign of terror." More than 600 tips had poured into the hotline by Monday.

But with the trail going cold, attention shifted to the incident which Dorner says triggered his fury with the LAPD.

Fresh out of the navy reserves, where he had served in Bahrain and Kuwait, Dorner, a trained marksman, was a rookie police officer on 28 July 2007, when he accompanied a training officer, Teresa Evans, to a reported disturbance at the Doubletree Hotel in San Pedro.

They encountered a mentally ill homeless man, Christopher Gettler. Evans tasered him after he threw a punch. Gettler was later returned to his father and that seemed to be the end of the incident.

Two weeks later, however, according to LAPD records published by the LA Times, Dorner told a superior, Sergeant Donald Deming of the harbour division, that after the tasering, Evans had kicked Gettler in the chest and face, snapping his head back and causing him to bleed.

An internal affairs investigation ruled that the kicking did not happen and that Dorner had lied. This led to a disciplinary hearing in 2008.

Dorner's attorney, Randal Quan, a former police captain, called the case against his client "very, very ugly" said he "wasn't given a fair shake." Quan said: "In fact, what's happening here is this officer is being made a scapegoat."

Dorner said he had not immediately reported the kicks because he had filed other complaints against fellow officers and feared retaliation.

Gettler's father, Richard, testified noting a slight puffiness around his son's face when police brought him home. His son told him he had been kicked by a police officer. The younger Gettler repeated that version of events under questioning from Quan, a video of which was obtained by Fox 11 News. These efforts on his behalf left Dorner unimpressed and angry with Quan.

Gettler, who, according to his father, is prone to contradictions, described the kicker as female and "almost black" with dark hair, before correcting himself to say light hair. Police records describe Evans as white with blond hair.

She denied kicking Gettler, and said Dorner had trouble adjusting to his new job as a police officer. One day, he cried in their patrol car, she said. Dorner made his complaint about Evans a day after being told that her evaluation said he needed to improve his performance.

Three witnesses – two hotel employees and a port police officer – testified that they did not see the kicks. Dorner was dismissed. During an appeal in 2010 a county superior court judge, David Yaffe, concluded he was "uncertain whether the training officer kicked the suspect or not", according to court records cited by the Times. Nevertheless, the judge upheld the sacking.

The dismissal stung Dorner as unjust, and an example of LAPD racism, according to a 14-page "manifesto" he posted online on January 31. He said he would keep killing until the truth was known about the case.

On Saturday, a week after Dorner's alleged rampage began, LA police chief Charlie Beck ordered a review of the case. "I do this not to appease a murderer. I do it to reassure the public that their police department is transparent and fair." Members of the FBI and other forces will be part of the investigation.

Dorner's manifesto named about 40 targets for revenge. He allegedly began his rampage on February 3 by targeting Monica Quan, 28, the daughter of Randal Quan, and her fiance Keith Lawrence, 27. They were shot dead in their car.

Four days later he exchanged shots with a patrol, grazing one officer in the head, and later allegedly ambushed two other officers at a traffic light, killing one, Michael Crain, 34, a veteran, and wounding the other, a trainee. A huge security operation is expected at Crain's funeral on Wednesday.

The hunt has focused on Big Bear after Dorner's pickup truck was found burning and abandoned on a dirt road, but there are statewide alerts in California and Nevada, with pilots and aircraft operators told to watch for stolen planes or suspicious passengers.

"It is believed that Dorner received flight training during his time in the US navy, but the extent of his potential flying skills is unknown," said a Transportation Security Administration notification.

"While there is no specific information at this time that Dorner is considering using general aviation, TSA requests that operators use an increased level of awareness concerning any suspicious activity during the coming days."