Police blocked off a street and stood guard in front of a home in a typically quiet Southern California suburb Sunday, protecting a man they believe has been targeted by a fugitive ex-police officer suspected of killing three people and setting the region on edge by eluding authorities in a sprawling manhunt that has lasted days.

Irvine residents, meanwhile, were left to adjust to life in the midst of a heavy police presence and wonder when things might return to normal.

'We will not tolerate this reign of terror.'—L.A. Mayor Antonio Villaraigosa

Authorities have been working to protect dozens of families in the area considered targets based on Christopher Dorner's Facebook rant against those he held responsible for ending his career with the Los Angeles Police Department five years ago.

Among those Dorner, 33, is suspected of killing is a Riverside police officer, and on the fourth day of the manhunt, authorities put up a $1 million US reward for information leading to his capture.

"We will not tolerate this reign of terror," said L.A. Mayor Antonio Villaraigosa.

Alleged Dorner sighting in L.A.

Police were flooded with tips after the $1 million reward was announced. Los Angeles police Lt. Andrew Neiman said around 600 clues were being investigated as of Monday morning.

One reported Dorner sighting prompted police to surround a Los Angeles home improvement store, but police spokesman Gus Villanueva could not immediately say whether the tip was legitimate.

After days without resolution, Dorner's fugitive status caused concern among some and downright fear among others in Irvine, an upscale community that the FBI consistently ranks among the safest cities in the U.S.

Christopher Dorner, a former Los Angeles officer, was fired from the LAPD in 2008 for making false statements and is suspected of killing three people. (Los Angeles Police Department/Associated Press)

"If he did come around this corner, what could happen? We're in the crossfire, with the cops right there," said Irvine resident Joe Palacio, who lives down the street from the home surrounded by authorities protecting a police captain mentioned in Dorner's posting.

"I do think about where I would put my family," he said. "Would we call 911? Would we hide in the closet?"

The neighborhood has been flooded with authorities since Wednesday. Residents have seen police helicopters circle and cruisers stake out schools. Some have responded by keeping their children home. Others no longer walk their dogs at night.

Served in Navy, earned rifle marksman ribbon

Dorner's background added to the anxiety. The former LAPD officer also served in the Navy, earning a rifle marksman ribbon and a pistol expert medal. He was assigned to a naval undersea warfare unit and various aviation training units, according to military records. In his online post, Dorner vowed to use "every bit of small arms training, demolition, ordnance and survival training I've been given" to bring "warfare" to the LAPD and its families.

As tense Irvine residents went on with their lives, police looked into a taunting phone call to the father of the woman they believe Dorner killed last week.

Two law enforcement officers who requested anonymity because of the ongoing investigation told The Associated Press they are trying to determine whether Dorner made the call telling retired police Capt. Randal Quan that he should have done a better job protecting his daughter.

The bodies of Monica Quan and her fiance were found shot dead last Sunday in Irvine, marking the start of the high-profile case.

Things escalated early Thursday morning, when police say Dorner got into a shootout with police in Corona, grazing an LAPD officer's head with a bullet before escaping. Authorities believe he then used a rifle to ambush two Riverside police officers, killing one and seriously wounding the other.

4th day of manhunt

About 105 kilometres away, the manhunt continued in the San Bernardino mountains near the ski resort town of Big Bear, where authorities found Dorner's burned out pickup truck Thursday. Police have since said they discovered weapons and camping gear inside the vehicle.

The search scaled down as the weekend went on, but a helicopter with heat-seeking technology scanned the area as two-dozen officers went back to some of the 600 cabins they earlier visited door to door.

Meanwhile, police responded to the Sunday afternoon sighting report in Los Angeles, establishing a command post and slowly evacuating a Lowe's Home Improvement store in the city's San Fernando Valley.

Villanueva emphasized that the major response was a precaution. He couldn't say whether Dorner was in the area.

With little apparent evidence pointing to Dorner's whereabouts, worrisome questions emerged: How long could the intense search be sustained? And, if Dorner continues to evade capture, how do authorities protect dozens of former police colleagues whom he has publicly targeted?

Costly protection details for targeted families

LAPD Cmdr. Andrew Smith said the department has deployed 50 protection details to guard officers and their families who are deemed targets in Dorner's manifesto.

"It can't be one guy with a gun in a living room," Smith said, suggesting that more officers would be necessary to keep families safe.

The truck authorities have identified as belonging to ex-LAPD officer Christopher Dorner was discovered burning on a road on the Bear Mountain Resort in Big Bear Lake California. (Gene Blevins/Reuters)

The department, however, is looking for alternatives if the search for Dorner stretches on, whether it's reducing the numbers of officers or something else, he said.

There were no plans to reduce protections until Dorner was in custody, said Los Angeles police Sgt. Rudy Lopez.

As long as Dorner's whereabouts are unknown, the police department must provide protection to those named in his rant, said Chuck Drago, a Florida-based police consultant.

"We realize it costs money and it gets expensive, but this is as clear of a threat as you can get," he said. "We know that if he's able to get to these targets then he's probably able to hurt them. The money is always an issue but not when it's somebody's life at stake."

If the search drags on, the LAPD will likely find safe houses for the targeted individuals, much as they would for witness protection participants, instead of posting officers outside their homes, Drago said.

LAPD remains on modified tactical alert, responding only to priority calls and not to those for lesser issues such as public intoxication or business disputes.

Authorities Sunday morning had six cars protecting Capt. Phil Tingirides, who chaired a disciplinary panel that stripped Dorner of his badge. Black and white police cruisers were posted on each end of his street and four more were parked outside his home. At least a half-dozen officers were visibly standing guard.

Meanwhile Palacio, who has to navigate the heavy police presence to get to and from his home, said his family is trying to keep routines normal.

"Life goes on," he said, "and we're doing our thing."