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 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. According to sources, Iraqi government officials were involved in the deal, apparently without the knowledge of the US Baghdad command — a departure from the usual pattern of US-overseen arms purchases. Why these officials resorted to “black” channels and where the weapons were headed is unclear.

Some guns the US 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 US-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 US-supplied guns are flowing from Iraq to anti-government militants on their soil.

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

Operation Parabellum began in 2005 as a routine investigation into drug trafficking by organised-crime figures, branched out into an inquiry into arms dealing with Libya, and then widened to Iraq.