Within hours after the huge car bomb exploded in the crowded town square here--killing 29 people who had come downtown to shop on a bright August Saturday--police had a fairly good idea who the culprits were. Just days later, their suspicions were confirmed when a splinter faction of the Irish Republican Army admitted responsibility.

And yet, as the people of this pleasant village at the center of Northern Ireland gather Sunday to mark the first anniversary of their town's darkest day, police records list the Omagh bombing as an unsolved crime. Although many people--including ranking members of the local government--appear to know who the killers are, nobody is in jail for the crime, and police seem to have reached a dead end after a year of intense detective work.

In one of the largest joint investigations in Irish history, police from the British province of Northern Ireland and from the Republic of Ireland to the south have questioned 4,000 people. They have traced the path of the maroon Vauxhall sedan that held the bomb, and tracked cellular telephone calls the driver made as he carried the 500-pound device to Omagh's town square.

About two dozen people have been arrested, some more than once. Yet all but one of those suspects have been released without charges. Colm Murphy, 46, a pub manager from Ireland, has been charged with conspiracy in the case, but he is currently free on bail and no trial is scheduled.

The difficulty of proving a criminal charge--in a situation where many people are sure they know the identity of the perpetrators--reflects the political and religious divisions that have plagued Northern Ireland for decades. The problems facing police in the Omagh case--divided loyalties and deep distrust--are the same basic problems that have sidetracked last year's Good Friday peace agreement and darkened the hope for a political solution to Northern Ireland's long-running "Troubles."

The car bomb that blew up in mid-afternoon last Aug. 15 killed grandmothers, schoolchildren and a woman pregnant with twins. It injured more than 350 people, leaving some maimed and others blinded. It destroyed the heart of Omagh's downtown. It infuriated the people and the political leaders of Britain and Ireland, who swore that those responsible would face swift and stern police action.

But to take action, the police need cooperation from people familiar with the killers. And that has been slow in coming.

The IRA and other paramilitary groups here hold to a code that requires that no member cooperate with police. Violators of the code are punished by beatings or murder.

On top of that, politicians affiliated with the IRA view the British-run police force in Northern Ireland, the Royal Ulster Constabulary (RUC), as a political adversary.

A group of anti-British hard-liners who call themselves the Real IRA have admitted detonating the Omagh bomb. The faction is small; police estimate no more than a few dozen people.

Numerous reports here say that the Real IRA is centered in Dundalk, Ireland, just south of the border. Its leader is said to be Michael McKevitt, who broke with the IRA when it backed the Good Friday agreement, which was designed to end nearly 30 years of sectarian warfare and give Catholics additional political power in the predominantly Protestant province.

At the time of the bombing, McKevitt lived in a quiet suburban home near Dundalk with Bernadette Sands-McKevitt. She heads an anti-agreement organization called the 32-County Sovereignty Committee, which argues that the 26 counties in Ireland and the six counties that comprise Northern Ireland must be reunited under the republic's flag.

Sands-McKevitt has been a successful fund-raiser among Irish Americans, and some of the U.S. money she brings home pays for the Real IRA's deadly arsenal, according to IRA-watchers here.

Both McKevitts have denied involvement in the bombing.

Officials of Sinn Fein, the political party connected to the IRA, are familiar with the Real IRA, and seem to know which members were involved in the Omagh bombing. But these people have refused to share any of their knowledge with the police.

"There's no question that the top people in Sinn Fein . . . know who did this crime," said Michael Gallagher, father of an Omagh victim and co-chairman of the Omagh Support and Self-Help Group. "But they won't provide any evidence. They have responded to our pain with a wall of silence."

Martin McGuinness, the second-ranking member of Sinn Fein and an elected member of the Northern Ireland Assembly--a government post that pays about $50,000 per year--has all but conceded that he knows who planted the bomb.

Asked why the Omagh killers have not been punished, McGuinness reacted angrily. "But they didn't get away with it," he replied. "We in Sinn Fein mobilized against them. We've told the people responsible that this [killing] has to stop, and it has."

But if McGuinness is in touch with "the people responsible," why won't he tell the police who they are?

"I am not an informer," he said. "You can't ask me to confer legitimacy on the policing power. I have no respect for the RUC. I won't help them."

McGuinness will presumably maintain his silence on Sunday, when townspeople, relatives of the dead, and victims recovering from their injuries will hold a series of memorial church services followed by a cross-community ceremony on the rebuilt town square.

One of those present will be Victor Barker, a lawyer in Surrey, England, whose son was killed in the blast. "There are people in official positions who know the perpetrators, but won't take a single step to see them brought to justice," Barker said. "We can have memorial services forever, but you'll never have an atmosphere of peace as long as the people who did this are walking around free."