Detective Inspector Mick Hughes of the Homicide Squad said investigators had received a significant amount of information that led them to believe David Dick's older brother Jonathan was the man captured on CCTV in a lift lobby at the centre the day before the attack.

Not even his own family, who made a heart-wrenching plea for help to solve the crime on Tuesday, had recognised the man in the vision as Jonathan.

"You saw Carol [David and Jonathan's mother], John [their uncle] and Simon [their younger brother] in here yesterday," Detective Inspector Hughes said on Wednesday.

"They were so traumatised and of course, that's been compounded by the news that we're looking at him [Jonathan] as a person of interest.

"Not one of them had an inkling that this was a possibility, and even when we spoke to Carol this morning ... she's coming to terms with it, but I really don't believe she has any belief that it is.

"These are a really honest, open family," he said.

David, 36, was attacked, possibly with a sword, as he stepped out of a lift in the Westfield Doncaster car park about 6am last Friday.

He was found bleeding to death in the lift lobby minutes later by members of an exercise group, who tried to save him.

Jonathan, 39, had been living in Seymour and was reported missing by another uncle on February 6. He bought a home in the country town in Victoria's north in 2011, and neighbours said he was friendly, but kept to himself and lived alone.

"He comes and goes and he doesn't annoy anybody," Gerard O'Rourke, who lives across the street, said.

Nicole Jones lived next door to Jonathan and said younger brother Simon came to her house on Sunday looking for him.

"He just knocked on the door and kindly asked if we'd seen his brother," Ms Jones said. "He said they'd recently lost a member of their family and they were trying to contact him to tell him."

Ms Jones said detectives were at Jonathan's house late on Tuesday night searching the home.

Police believe Jonathan could have gone to NSW or Queensland. His father, whom he used to live with in nearby Wallan more than a decade ago, moved to Brisbane with his partner in 2004 and died there about six months ago.

Detective Inspector Hughes said it wasn't unusual for Jonathan to drive up to Queensland to visit his father before his death.

Jonathan, who has no criminal history police said, was recently diagnosed with a mental health condition.

"This may be an overwhelming situation to him. We're saying [to him] don't do anything to hurt yourself, you reach out to us," Detective Inspector Hughes said.

"Just remember, you've still got a family here regardless of the circumstances you may find yourself in."

He said the boys' mother believed the relationship between siblings had been good, with the youngest brother Simon being close to both.

"We've got no alarm bells there," Detective Inspector Hughes said.

"We don't know the motive."

Jonathan, who recently cut his hair short, may be driving one of two cars: a 1997 blue Ford Fairmont sedan, registration ONV 954; or a white 2002 Ford Falcon station wagon, registration RIL 435.

Images of Jonathan and the vehicles were released to the public in the hope someone may have information on his whereabouts for police.

David, a former Melbourne High student and president and captain of the Glen Iris Cricket Club, was attacked as he walked to the bus stop to travel into town, where he worked as a concreter.

Police said his normal routine was to walk from his home in Doncaster and cut through the shopping centre car park to catch the bus.

His attacker knew his routine and had been watching him, and was captured on CCTV in the same location, at the same time, the day before he was killed.

Detectives said they believe he was planning to kill David last Thursday, but for an unknown reason, backed out and returned on Friday.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or make a confidential report at www.crimestoppersvic.com.au