Detectives investigating the murder of former loyalist leader Jim Gray have arrested six people, police have said.
Police kept the area of the killing cordoned off
Detective Superintendent George Hamilton said the arrests followed a number of searches.
He said that the involvement of the Ulster Defence Association was a "major line of inquiry" in the investigation.
Gray, 47, the flamboyant former leader of the UDA in east Belfast, was shot outside his father's house on the Clarawood estate in the city.
Gray was expelled from the illegal organisation last March.
He was recently released on bail on charges of money laundering, and was living at his father's home in Knockwood Park while awaiting his court appearance.
He was shot behind a car parked outside the house on Tuesday night at about 2000 BST.
DS Hamilton said Gray had been warned that he was under threat since his release on bail.
Jim Gray had been recently released on bail
"Police have complied with their obligations under the European Convention on Human Rights to advise Mr Gray of the threats against him and provide him with personal security advice."
"This was the brutal killing of a man by another human being - in any civilised society that is unacceptable," said DS Hamilton.
The four men and two women detained following Gray's death are being questioned at the serious crime custody suite in Antrim.
DUP East Belfast MP Peter Robinson said there was no excuse for the murder.
Frankie Gallagher from the Ulster Political Research Group, which advises the UDA politically, said while the murder should be condemned, there were other concerns.
"I am not surprised. My sympathy goes to his immediate family," he said.
"What the man done and how he terrorised people... blame cannot be laid at the door of his father or his immediate family, and my sympathy goes to them."
In April, just over a week after being expelled from the UDA leadership, Gray was stopped by police near Banbridge, County Down.
He was travelling in a car towards the Irish border, and police suspected he was trying to leave the country.
The police found a bank draft for 10,000 euro and nearly £3,000 in cash in his car.
Gray claimed the money had come from the sale of two pubs in east Belfast.
However, police believed it was obtained through crime including extortion and drug dealing.
He was charged with money laundering and possessing the proceeds of crime and was remanded in custody.
As the police investigation continued, detectives seized more than 100,000 documents and raided council offices, planning offices and premises used by solicitors, estate agents and accountants.
He continued to apply for bail which was granted last month on condition that he lived at the address where he was shot on Tuesday.
Chief Constable Sir Hugh Orde said the PSNI had objected in the High Court to Gray's bail application.
"He was not under surveillance, he was not under police protection: he was a suspect, not a witness, and he chose to carry out his lifestyle as he saw fit," he said.
Gray had a reputation for dressing flamboyantly and wearing heavy gold jewellery.
This earned him a number of derisory nicknames including 'Doris Day' and the 'Brigadier of Bling', while the group surrounding him were branded the 'Spice Boys'.
Northern Ireland Officer minister David Hanson condemned those responsible for the killing.
"Whoever it is, the government's commitment is to tackle that criminality to make sure that we deal with that criminality in a wider sense, to make sure that we take on board the type of issues to make that type of murder stop occurring," he said.
DUP deputy leader Peter Robinson said there was no excuse for the killing.
"Those who take the law into their own hands have nothing to contribute to society," the East Belfast MP said.
"There is no excuse for acting as judge, jury and executioner."
Ulster Unionist leader Sir Reg Empey said it was "clear that there are still those in society who want to stick to the old ways of doing things".
"The task of re-establishing the ascendancy of the democratic process is far from over," he said.
SDLP assembly member Alex Attwood has also condemned the shooting.
He said he hoped the situation would not escalate to a point where others, including innocent people, might be killed or injured.
In September 2002, Gray was shot in the face as he arrived at a house in the Garnerville area of east Belfast.
At the time, police said the shooting was "loosely related" to the murder of Loyalist Volunteer Force leader Stephen Warnock, who was shot dead as part of a feud between loyalist paramilitaries.