BELFAST, Northern Ireland – Although they’ve committed the world’s biggest all-cash robbery in peacetime, the audacious raiders of a Belfast bank face an uphill struggle to use most of their $42 million haul – because their Northern Ireland-produced bills are easy to track and hard to spend.

“These guys saw bounty beyond their wildest dreams and loaded up on everything. But a lot of it is effectively toilet paper,” said an expert on money laundering and international crime, Jeffrey Robinson, about Monday’s raid on the central cash vault of Northern Bank.

More than $25 million of the bills stolen were newly minted notes produced by Northern Bank itself. Most of the rest, police say, are used notes produced either by Northern or the other three major banks in this British territory. While Northern Ireland-issued currency is officially British pounds sterling, other parts of the United Kingdom usually refuse to accept it – and most of the rest of the world barely recognizes the stuff.

Now, because the size of the robbery has attracted much attention, the entire global financial community is on alert for large volumes of used Northern Ireland notes.

“They should have been much more selective in what they stole. But they had sugar plums dancing in their heads, they were stupid, and they went for everything,” said Robinson, who thinks the only notes that the gang can launder safely outside Northern Ireland will be those produced by the Bank of England, Britain’s mainstream note.

Further complicating the crooks’ hopes of realizing their profits, any attempt to spend large amounts of cash in Britain and Ireland can attract attention from powerful anti-racketeering agencies with power to seize the property and other assets of criminals.

The 45-member detective team trying to hunt down the gang, meanwhile, rejected criticism yesterday of the police handling of the case, but made two more shocking admissions: They’d missed by minutes a chance to stop the robbers’ cash-filled van, and still don’t have a single serial number for any of Northern Bank’s factory-fresh cash.

Detective Superintendent Andy Sproule admitted police couldn’t yet advise businesses in Northern Ireland and farther afield in Scotland and the Irish Republic – where Northern Ireland-is sued notes are sometimes accepted – what serial numbers to look out for.

This meant police couldn’t be sure whether some stolen cash had already been used in transactions, he said.

He also confirmed that two members of the public spotted the robbers’ white van Monday night parked in an alley outside the bank’s entrance for armored cars, alongside two suspicious looking men who appeared to be wearing wigs. These two people, he said, told a passing traffic-ticket official, who immediately called police at 8:13 p.m.

But by the time a police unit arrived five minutes later, the van had left. “There was no evidence of a crime here; the gates were closed,” he said.

Mr. Sproule said police urgently wanted to interview both civilian witnesses, neither of whom have been identified.

Police didn’t find out about the robbery until three hours later, after the gang released the loved ones of two key Northern Bank officials it had been holding hostage – and threatening to kill – if either executive refused to override the bank’s security measures and clean out the vault.

The outlawed Irish Republican Army, the most sophisticated of Northern Ireland’s rival paramilitary groups, issued a one-line statement yesterday denying any involvement in the robbery.

Police discounted the denial. They said an IRA unit was one of five Northern Ireland gangs on its list of most likely suspects, alongside a unit of the major outlawed Protestant group, the Ulster Defense Association; the Irish National Liberation Army, a smaller rival of the IRA based in hard-line Catholic areas, and two other gangs with no political affiliations.

Mr. Robinson, the money-laundering expert, said the IRA – which runs a range of illegal rackets and smuggling operations and takes cuts from other crime gangs that operate in Catholic areas – was probably either directly or indirectly involved.

“The IRA is the largest criminal organization in Northern Ireland. The IRA runs parts of Northern Ireland. They either knew about it and sanctioned it, or didn’t know about it and will be going after these guys to demand their cut,” he said. “Either way, they will be demanding hard currency. You can be sure they’ll be sticking the little guys with the Northern Bank notes.”