The death of a man killed in a "vicious, targeted attack" at a city apartment on Tuesday night is being treated as a major crime.

Detective Inspector Brett Featherby, acting officer in charge of Major Crime said a friend of the victim - Adelaide father, 46-year-old Nathan Russell - contacted police after discovering his body in unit 12 of an apartment block on Sturt Street.

"A friend of the deceased had attended that address, after trying to contact him, and when he failed to raise him, he has attended there, and found him deceased," Det Insp Featherby said.

Police and detectives swarmed the Housing SA block about 6.30pm Tuesday finding the body inside the apartment.

"From the investigation today (Wednesday), we believe the assault on Nathan was actually a targeted attack.

"Nathan was targeted at that address, and he was subjected to a vicious attack, which has unfortunately caused his death."

For operational reasons, Detective Inspector Featherby could not say how Mr Russell was murdered, but said multiple items have been seized from his home for investigation.

A number of avenues of inquiry are underway including whether drugs were involved in the incident, but Detective Inspector Featherby said they have no firm suspects.

He said police believe people who were in the vicinity of the crime may know what happened, and the circumstances that led to the death.

"We're seeking anyone with information regarding his death to come forward, and to do the right thing by his family, and assist the police investigation.

"I ask that anyone who saw anyone acting suspiciously between the evening of 25th January, and 6.28pm on the 26th January to contact police."

Investigators will be available on the Crime Stoppers line from 5pm to 9pm Wednesday. Anyone with information should call 1800 333 000.

Friends of Mr Russell gathered to pay their respects and comfort one another on Wednesday morning.

The man's heartbroken mates stood out the front of the apartment block, consoling each other and hugging.

Lighting a candle in their mate's honour, they said he was a kind man who would always be there when they needed him.

A woman who lives in the apartments, who did not wish to be named, said a man drove past on Sunday afternoon and threw an object at a window on the second floor, smashing a window.

She said she heard arguing in the block about 5.30pm Tuesday, an hour before the police were alerted to the death.

But Jenny Hwang, a worker at Donburi House, a restaurant next door to the scene, said she did not hear much from the block while at work on Tuesday night.

"Police cars kept coming … but we didn't know (why) because we were so busy," Ms Hwang said.

Ms Hwang felt sad a man had died.

Detectives from Eastern District CIB and Major Crime Investigation Branch, along with Forensic Crime Scene examiners, have been at the scene most of Tuesday evening, likely to remain at the scene most of Wednesday.

A Sturt Street resident who did not wish to be named said he came home to discover police in the street and knew something serious was wrong.

"I just thought 'there must be a death here, crime scene investigators are here', it's not good," he said.

A man was charged with brutally murdering a woman in the same street in 2019, allegedly stabbing the victim 80 times.

Numan Mohammed, 22, pleaded not guilty to the murder charge in the Adelaide Magistrates Court in October last year. Mohammed will stand trial in the Supreme Court in February.

Anyone with information that may assist police is asked to contact Crime Stoppers on 1800 333 000 or online at www.crimestopperssa.com.au.

Originally published as Nathan Russell died in a 'vicious, targeted attack' in CBD