President Bush's envoy to Northern Ireland (search) called Wednesday for the IRA to disband after the outlawed group made an unprecedented public offer to kill four men — including two of its own expelled members — linked to a Belfast slaying.

The envoy, Mitchell Reiss (search), told BBC radio in Belfast that the IRA's allied Sinn Fein (search) party should accept the legitimacy of the police force.

"It's time for the IRA to go out of business. And it's time for Sinn Fein to be able to say that explicitly, without ambiguity, without ambivalence, that criminality will not be tolerated," Reiss told the BBC in the interview from Washington.

The Irish Republican Army (search), which is supposed to be observing a 1997 cease-fire in support of Northern Ireland's peace process, has faced weeks of embarrassment over its members' role in killing a Catholic civilian, intimidating witnesses and destroying evidence. The case highlights the IRA's decades-old practice of seeking to impose its authority on the most hard-line Catholic parts of Northern Ireland.

The victim's family, which lives in an IRA power base in east Belfast, has waged a rare public campaign demanding that the IRA acknowledge its involvement in killing Robert McCartney (search), 33, and encouraging witnesses to give evidence to police.

The McCartneys' stand has forced the IRA to make a string of admissions, culminating in Tuesday night's declaration it had offered to kill four people the IRA blames for the Jan. 30 killing outside a Belfast pub.

"The IRA representatives detailed the outcome of the internal disciplinary proceedings thus far and stated in clear terms that the IRA was prepared to shoot the people directly involved in the killing of Robert McCartney," according to a statement from the group.

The McCartneys dismissed the IRA statement as irrelevant to their needs — to get the killers and accomplices convicted in a court.

They also accused the IRA of continuing to understate its involvement in the crime, citing confidential witness claims that up to a dozen IRA members held the pub patrons hostage while they swabbed up evidence. McCartney was initially attacked inside the pub, then had his neck and stomach fatally slashed outside it.

"It was that cover-up which prevented those who murdered Robert from being brought to justice," said a statement read by Claire McCartney, one of his sisters.

The British and Irish prime ministers, Tony Blair (search) and Bertie Ahern (search), both denounced the IRA's offer as bizarre.

"It's an extraordinary statement and a shock to the system," Ahern said in Dublin.

Reiss specifically chided Sinn Fein leader Gerry Adams (search) for his remarks during his speech Saturday to the party's annual conference. Adams, reflecting traditional IRA-Sinn Fein policy, claimed the movement wouldn't tolerate criminals in its ranks. He immediately qualified that position, arguing that the IRA wasn't committing crimes when it broke laws "in pursuit of legitimate political objectives."

Analysts say that view justifies virtually all of the IRA's activities, including its robberies, fuel smuggling and policy of maiming petty criminals within the IRA's Catholic power bases.

Reiss said he found that comment "particularly worrisome. ... You can't sign up for the rule of law a la carte."

Ahern said the IRA had a history of using death threats as a way of maintaining order. "But when you actually see it in written form ... it's horrific," he said.

"The IRA statement yesterday frankly defies any description," Blair told the House of Commons in London.

Blair said the IRA had revealed why both governments and every other political party in both parts of Ireland were demanding the IRA fully disarm and disband.

"We have made considerable progress in Northern Ireland," he said, referring to the peace process and the Good Friday peace pact of 1998. "But we now have an impasse because of the refusal of the IRA to give up violent activity of whatever sort."

Detectives trying to bring charges against McCartney's killers said they have arrested a man on suspicion of involvement. The man appeared voluntarily at a Belfast police station, accompanied by a lawyer.

Nobody has been charged. Police say 10 people previously arrested on suspicion of involvement have refused to make any statements while in custody and been released without charge.

Northern Ireland's police commander, Chief Constable Hugh Orde, said he didn't need the IRA to tell his officers who killed McCartney. He did need members of the public to feel safe enough to testify — because the IRA traditionally kills anybody who informs on IRA activities to police.

"We know the names of the suspects. Many people claim to be supplying those, but police work has identified those responsible," Orde said. "What the police need are witnesses willing to come forward ... and give evidence in court. That's the way the law works."

Other politicians accused IRA-Sinn Fein leaders of continuing to cover up the movement's full role in the killing. The IRA statement Tuesday said just four people, including two IRA members, were involved in the killing; previously the IRA has expelled three members allegedly involved, while Sinn Fein has suspended seven members.