BAGHDAD, Iraq – One day after five Britons were snatched in broad daylight from a government building by men operating under the cover of a police squadron, the capital was left Wednesday trying to grasp how another bold abduction could be carried off without a shot fired.

Despite a massive search operation by U.S. and Iraqi forces, there was no sign of the missing Britons.

Iraqi authorities arrested a number of security guards who stood by while dozens of gunmen went into the Finance Ministry office near the slums of Sadr City and demanded, “Where are the foreigners?”

Iraqi political factions traded recriminations over how the foreigners could be walked out of their workplace and driven off in sport-utility vehicles, which appeared to have been commandeered from Iraqi police. Adding to the confusion, a top aide at the Iraqi Finance Ministry said the Westerners, one economic adviser who was contracted by the U.S. government and four security guards hired by his firm, were in a place they’d been warned not to go.

No group claimed responsibility for the abduction.

The abductors struck shortly before noon Tuesday, bursting into two British consultants’ lecture on information technology, a senior Finance Ministry official said.

The official said at least one of the Britons evaded the intruders and avoided abduction. The apparent sophistication of the plot fed speculation that central government or police officials had eased the way for the 40 or so kidnappers, who arrived in about 10 vehicles and carried authentic-looking paperwork.

Iraqi Foreign Minister Hoshyar Zebari noted long-standing suspicions that sectarian militia members had infiltrated security forces, and the scale of Tuesday’s operation suggested the attackers had used inside connections to carry out their plot.

“This was a . . . calculated, organized operation, and a sophisticated one,” Zebari said.

“It has all the hallmarks of a strong militia.”

Sixteen government guards were on duty during the kidnapping and did nothing to stop it. Several of those guards were arrested Wednesday, although it was unclear how many were detained for failing to thwart the kidnapping.

“It’s a sensitive government building and there was no resistance,” Zebari said. “That’s why one is suspicious. The location of the building gives you an indication, because not everyone can operate there except for certain people,” he said, referring to Sadr City, a stronghold for the Mahdi Army, the biggest Shiite militia.

was kidnapped in the Tuesday raid.