A 43-year-old father of one was shot twice in the chest during what police have described as a cold-blooded execution.
Joe Reilly was gunned down at his home in the nationalist Poleglass area of west Belfast on Thursday night.
A motive for the shooting has yet to be established.
PSNI Detective Chief Inspector Peter Montgomery, who is leading the murder hunt, said: “It really is difficult to know how to describe what happened last night.
“The adjectives ‘brutal’, ‘senseless’ and ‘barbaric’ are inadequate.
“This was a cold-blooded summary execution of a man in his own house.”
It is understood the killers, one armed with a handgun, entered the house at Glenwood Court through an open door.
They ordered two other people to lie down on the kitchen floor while Mr Reilly was held in the living room where he was shot.
He died at the scene, police said.
There has been widespread speculation that dissident republicans may have been responsible for the killing but, as yet, no group has claimed responsibility.
Mr Montgomery said the victim had been known to police but added: “I will keep a completely open mind as to why he has been killed.
“Mr Reilly had a criminal record and that will form part of my investigation.
“But what is important here to understand is that a man has been brutally murdered in his house and I am determined to try and find out who was responsible and bring those people to justice and make Belfast a safer place to live.”
The 6ft tall gunmen, who were dressed in dark coats and trousers, fled through the front door of the house and ran down the street.
One of the suspects was also wearing dark rimmed glasses and a Celtic scarf.
Police want to speak anyone who was in the Glenwood Court, Glenwood Crescent or Laurelbank area before or after 8pm on Thursday and who may have seen anything.
However, detectives are particularly keen to speak to three children who were in the area after the shooting and may have seen the gunmen make their escape.
Mr Montgomery added: “My knowledge of what happened and why it happened is far from complete and any information people can provide to me is very important.”
An incident room has been set up at Musgrave Street PSNI station from where a team of detectives are working on the case.
Meanwhile, political leaders have united in condemnation of the murder.
In a joint statement, the First and Deputy First Ministers Arlene Foster and Martin McGuinness said: “We unreservedly condemn the murder of a man shot dead in his home in Poleglass last night.
“This savage killing is senseless and has achieved nothing. The perpetrators care only for themselves.”
Northern Ireland Secretary James Brokenshire has also expressed outrage.
He wrote on Twitter: “Appalled by senseless and brutal murder in west Belfast last night. My thoughts are with family & everyone affected by this.”
It was the second shooting in the Poleglass area in less than a week.
On Sunday a man in his 50s was hit up to six times in the lower legs.
Justice Minister Claire Sugden described the murder as an attack on the whole community.
The independent MLA said: “I unreservedly condemn this appalling act and I offer my deepest sympathies to the family.
“Murder is an attack against our entire community and those responsible have no place in society while they continue to be involved in crime.”
Sinn Fein Assembly member Jennifer McCann, who was at the scene a short time after the shooting, appealed for anyone with information to contact the PSNI.
She said: “This brutal killing has achieved nothing other than plunging another family into grief and despair.
“I have expressed my condolences to the victim’s family and I appeal to anyone with information to bring it forward to the police.”
SDLP MLA Alex Attwood said such “barbaric” attacks had “no place in our past and they have no place in our society today”.
He said: “The voice of our condemnation has to be consistent and strong. At the scene last night there were many young people who, unlike previous generations, will not have witnessed this brutality. We must say to younger generations and to all, that an assault on the rule of law, an assault on a citizen, is an assault on us all.”
Alliance Party justice spokesman Trevor Lunn said the gunmen must be caught.
He said: “The local community will be rightly shocked by this murder.
“Those responsible need to be removed from the streets and should face the full rigours of the law. I would encourage anyone with information on this incident to pass it to police immediately.”
Anyone with information is asked to contact police at Musgrave Street PSNI station on 101.