The gun used by David Bieber to kill Pc Ian Broadhurst may have been part of a batch exported using bogus documents, leading charities have claimed.
Charities have called for tighter government gun controls
The HS-95 9mm pistol was part of a consignment licensed for export from Croatia to a US company in 1998, say Oxfam and Amnesty International.
Sources close to the inquiry told them bogus export papers were used.
Amnesty said investigators across Europe had recovered only 146 guns from the total shipment of 2,750.
The two charities plus the International Action Network on Small Arms have joined together in the Control Arms campaign.
Amnesty, which has called for tighter restrictions on the gun trade, claimed the weapons had been used in crimes including murder and armed robbery.
An Amnesty spokesman said: "We understand from sources close to the investigation that the gun used was part of a batch of weapons, licensed for export from Croatia.
"Confidential documents appear to show that 2,750 weapons were licensed for sale from Croatia to 'front' companies based in the United States, British Virgin Islands and Nigeria.
"However, these companies were reportedly not the true recipients of these weapons and investigators believe they are now in the hands of criminal networks in western Europe."
The charities have claimed such weapons could have been prevented from entering the criminal underworld if governments had agreed on an arms treaty.
Such a treaty would place obligations on all countries to ensure weapons are not sold using false paperwork and check legitimacy of all arms buyers, they said.
Kate Allen, of Amnesty International, said: "Countless murders are happening every day with illegal guns like the one used to kill Pc Broadhurst.
Tighter gun controls
"That more than 2,500 guns from this consignment are still in circulation is deeply worrying.
"Controls over the arms trade are so lax that these weapons were sold without any adequate checks."
And Oxfam's Anne MacDonald said: "Arms such as these are fuelling conflict, crime and misery all over the planet at an alarming rate.
"Governments must now work to control the deadly trade behind this killing and sign up to an international arms trade treaty."
In September, Foreign Secretary Jack Straw pledged his support for such a treaty, the charities said.
Charities are now calling for Labour to make an election manifesto pledge that such a treaty will be pursued.