British security chiefs appealed Monday for public help to catch the Irish Republican Army dissidents responsible for gunning down two soldiers — a hunt that challenges Catholics to inform on their own as never before.

Sinn Fein, the IRA-linked party that represents most of the Irish Catholic minority here, faced concerted pressure from other parties to take the next, painful step away from past beliefs and tell police about the Real IRA die-hards sheltering in the most hard-line Irish nationalist districts of this British territory.

Police investigating Saturday's killings — the first of British security personnel in Northern Ireland since 1997 — conceded they would have difficulty catching and prosecuting the two gunmen and their getaway driver unless the killers' neighbors break Northern Ireland's traditional code of silence on such matters. Any who do risk attacks against them and their families.

"We need public support. We will do everything in our power to protect those people who come forward," said Chief Superintendent Derek Williamson, the policeman leading the investigation. He said Real IRA sympathizers were likely harboring the killers somewhere in a Catholic district of Northern Ireland.

Sinn Fein lawmaker Gerry Kelly, who once led IRA car-bomb attacks in London and the biggest prison escape in British history, said many in the party's grass roots understood they needed to cooperate with the police as part of Northern Ireland's peaceful future. But they still considered helping the police to arrest IRA dissidents a bridge too far.

"Within the republican psyche there's an aversion to the whole idea," said Kelly, who pledged that Sinn Fein leaders were already challenging their own supporters to isolate Real IRA activists — if not finger them to police.

"We will support the police's investigations into this. And we will do it actively, speaking publicly," he said.

Protestant skepticism about Sinn Fein's commitment to enforcing law and order in Northern Ireland has stunted the development of the province's joint Catholic-Protestant administration. The four-party coalition gained office in 2007 but has achieved little of substance amid incessant squabbling — particularly about justice powers.

Sinn Fein wants to help exercise control over the police and courts, but the Protestant side insists those powers should stay in British government hands in London until Sinn Fein goes beyond saying they support the police.

Leaders of the Protestant side of the coalition emphasized Monday that they must see hard evidence of Sinn Fein's changed intent, with tip-offs that lead to arrests of the Real IRA unit responsible for Saturday's attack.

Nigel Dodds, whose Democratic Unionist Party represents the British Protestant majority, said Sinn Fein and IRA members must know plenty about the current membership of their Real IRA rivals.

The splinter group was founded 12 years ago by hard-liners who opposed the IRA's decision to cease violence and clear the way for Sinn Fein to negotiate with Britain and Protestant leaders. Those talks produced the Good Friday peace accord of 1998 and led, eventually, to IRA disarmament and Belfast's 22-month-old experiment in power-sharing.

"The test will be how much people in the republican community are urged, and actually come forward, with information about these killings," said Dodds, who is finance minister in the power-sharing coalition. "Because there's people out there who undoubtedly know who these people are, and they need to be seen to take action and give that information to the police."

Sinn Fein and the Democratic Unionists remain determined to maintain a common public front despite the killings. Democratic Unionist leader Peter Robinson and Sinn Fein deputy leader Martin McGuinness, the top two power-sharing figures, announced they will start a joint weeklong tour of the United States on Tuesday — two days later than planned because of the Real IRA bloodshed.

Police revealed new details Monday about the attack as hundreds of civilians delivered floral bouquets to the bullet-pocked, bloodstained scene of the crime: the entrance of the Massereene army base in Antrim, a mostly Protestant town west of Belfast that houses Royal Engineers training for deployment in Afghanistan.

Williamson said police studies of TV surveillance footage had documented how two masked Real IRA men armed with assault rifles waited in bushes across the road as off-duty, unarmed soldiers walked out of their fort to collect pizzas from two Domino's Pizza couriers. He said the attackers fired more than 60 bullets in about 30 seconds, closing quickly on foot to fire rounds point-blank at the prone victims.

He said police did have one excellent potential source of evidence that does not require public support: the gunmen's getaway car. The Real IRA appeared to have tried to set it on fire, to destroy forensic evidence, but failed.

Police have yet to inspect the abandoned vehicle in fear that the Real IRA might have booby-trapped it, an old IRA trick. Army explosives experts spent Monday slowly probing the vehicle using a remote-controlled robot.

In its statement of responsibility the Real IRA said it deliberately shot the Domino's workers — a 19-year-old Antrim man and a 32-year-old Polish immigrant — because they were British "collaborators" providing food to the enemy. During its own 1970-1997 campaign, the IRA also targeted civilians who did any business with the police or army.

The British army revealed the identities of the two dead soldiers: Cengiz Azimkar, a 21-year-old Londoner with mixed Irish and North Cypriot parentage, and Mark Quinsey, 23, from the English city of Birmingham. They were both supposed to depart Sunday for Afghanistan, and the pizzas were to be their final non-army rations for six months.

Brigadier George Norton, the senior army commander responsible for more than 4,000 Northern Ireland-based soldiers, spoke to journalists Monday at the scene of the shooting.

Norton rejected the idea that the army had grown careless to permit soldiers to collect fast-food deliveries in a publicly visible pattern. This violated the army's own decades-old security advice to deter IRA attacks — that "repetition kills" because it permits the IRA to identify patterns in soldiers' behavior and identify when they are most vulnerable.

"The army is living in Northern Ireland as part of the community. We have to lead as normal a life as possible. And ordering pizzas on an evening is something that everybody does around the community," Norton said. "Was it an avoidable routine? I don't think so."

He also dismissed criticism of the base's armed guards, who are employed by a private security firm. They did not fire at the dissidents. The brigadier said they could not have shot without adding to the wounds of the soldiers and pizza delivery men.

"Are you suggesting that people should have fired into a closely packed group, including my soldiers?" Norton said.