Four Jordanians detained in Kosovo over a gunfight between international police on Saturday are now being interrogated as potential suspects.
It is not yet clear what provoked the gunfight between colleagues
UN officials have confirmed that the incident began when Jordanian police fired on vehicles carrying US police.
The US officers returned fire and two Americans and one Jordanian were killed and 11 others were wounded.
The Jordanians could not be formally interviewed until the UN waived their right to diplomatic immunity.
UN officials have denied rumours that a quarrel about the war in Iraq sparked the gunfight.
"As far as we know, there was no communication between the officer who fired and the group of victims," AP news agency quoted UN spokesman, Neural Singh, as saying.
The shooting came a day after King Abdullah of Jordan said during a visit to the United States that the war in Iraq had created unprecedented animosity towards Americans across the Middle East.
The UN uses more than 3,000 international police to maintain order between Kosovo's Serbs and Albanians.
But it has confirmed no Serbs or Albanians were involved in the latest shoot-out, which took place in the compound of a prison in Mitrovica.
Special police unit
The incident occurred as the US officers left the detention centre after undergoing initial training. They had only been in Kosovo for 10 days.
The UN says that as they left the compound, at least one member of a Jordanian special police unit opened fire on the US vehicles.
The Americans returned fire. Two female Americans and one male Jordanian died.
UN police commissioner in Kosovo Stefan Feller said that among those killed was a member of the prison's management staff and a member of the special police unit.
Two of the injured are in a serious condition. Four Jordanian officers are being held as witnesses to the incident.
'Drop the gun!'
Doctor Milan Ivanov told the BBC that wounds to the injured were predominantly in the chest and abdomen and "were caused by firearms and possibly explosive devices".
A witness to the incident quoted by the Associated Press news agency said he heard gunshots followed by the cries of American officers yelling, "Drop the gun! Drop the gun!"
The BBC's Nick Hawton in Pristina says the incident could put fresh pressure on the UN, which has also been accused of falling behind in its plan for political reform in the province.
The top UN official in Kosovo, Harry Holier, said he was "deeply shocked and dismayed" over the incident.
The UN took charge of the Serbian province of Kosovo after an 11-week air war, in which Nato forces aimed to push back Serbian troops accused of war crimes against the ethnic Albanians.
The divided town of Mitrovica was the scene of fierce fighting between the two ethnic groups in March.