PERUGIA, Italy — In a hidden corner of Rome’s busy Fiumicino Airport, police dug quietly through a traveler’s checked baggage, looking for smuggled drugs. What they found instead was a catalog of weapons, a clue to something bigger.

Their discovery led anti-Mafia investigators down a months-long trail of telephone and e-mail intercepts, into the midst of a huge black-market transaction, as Iraqi and Italian partners haggled over shipping more than 100,000 Russian-made automatic weapons into the bloodbath of Iraq.

As the secretive, $40 million deal neared completion, Italian authorities moved in, making arrests and breaking it up. But key questions remain unanswered.

For one thing, the Associated Press has learned that Iraqi government officials were involved in the deal, apparently without the knowledge of the American Baghdad command — a departure from the usual pattern of American-overseen arms purchases.

Why these officials resorted to “black” channels and where the weapons were headed is unclear. The purchase would merely have been the most spectacular example of how Iraq has become a magnet for arms traffickers and a place of vanishing weapons stockpiles and uncontrolled gun markets since the 2003 American-led invasion and the onset of civil war.

Some guns that the American government bought for Iraq’s police and army are unaccounted for, possibly fallen into the hands of insurgents or sectarian militias. Meanwhile, the planned replacement of the army’s AK–47s with American-made M–16s may throw more assault rifles onto the black market. And the weapons free-for-all apparently is spilling over borders: Turkey and Iran complain American-supplied guns are flowing from Iraq to antigovernment militants on their soil.

Iraqi middlemen in the Italian deal, in intercepted e-mails, claimed the arrangement had official American approval. An American spokesman in Baghdad denied that.

“Iraqi officials did not make MNSTC–I aware that they were making purchases,” Lieutenant Colonel Daniel Williams of the Multi-National Security Transition Command-Iraq, which oversees arming and training of the Iraqi police and army, told the AP.

“Operation Parabellum,” the investigation led by Dario Razzi, anti-Mafia prosecutor in this central Italian city, began in 2005 as a routine investigation into drug trafficking by organized-crime figures, branched out into an inquiry into arms dealing with Libya, and then widened to Iraq.

Court documents obtained by the AP show that Mr. Razzi’s break came early last year when police monitoring one of the drug suspects covertly opened his luggage as he left on a flight to Libya. Instead of the expected drugs, they found helmets, bulletproof vests and the weapons catalog.

Tapping telephones, monitoring e-mails, Mr. Razzi’s investigators followed the trail to a group of Italian businessmen, otherwise unrelated to the drug probe, who were working to sell arms to Libya and, by late 2006, to Iraq as well, through offshore companies they set up in Malta and Cyprus.

Four Italians have been arrested and are awaiting court indictment for allegedly creating a criminal association and alleged arms trafficking — trading in weapons without a government license. A fifth Italian is being sought in Africa. In addition, 13 other Italians were arrested on drug charges.