Relatives of those killed in the MH17 air disaster Wednesday urged Dutch Prime Minister Mark Rutte to launch a global campaign to obtain radar images which may help pinpoint who fired a missile at the plane.

All 298 passengers and crew onboard the Malaysia Airlines flight jetliner -- most of them Dutch -- died when it was shot down en route to Kuala Lumpur over war-torn eastern Ukraine in July 2014.

Officials with the Dutch safety board (OVV) concluded in their investigation last year that the Boeing 777 was hit by a Russian-made BUK missile fired from territory held by pro-Russian rebels. But they did not say definitively who pulled the trigger.

A separate criminal investigation is also underway in the Netherlands to try to find those responsible and bring them to justice.

Now the Dutch relatives have written to Rutte to protest that primary radar data was not made available to the investigators by either the Russian or Ukrainian authorities.

"We can't accept that people have refused to provide crucial information," the families said in the letter, dated Tuesday and published Wednesday.

They are urging Rutte to push either the United Nations or the International Civil Aviation Organisation (ICAO) to demand that the data is handed over.

The Dutch investigators acknowledged that while they were given certain radar data by both Kiev and Moscow, they had not received what is called the "primary radar" from either country.

Primary radar, unlike secondary radar, can detect aircraft or unknown flying objects even if they are not equipped with a transponder, according to the ICAO.

That means primary radar could possibly trace the trajectory of a missile, for example.

The Dutch investigators said in their October report that it was "very unlikely" that the primary radar systems in place could have detected the missile. But the families contended that that finding did not rule it out completely.

Under the ICAO regulations primary radar data should be kept for 30 days, and longer if it is needed for a crash inquiry.

But Russian officials told the Dutch team they had not kept the data because the disaster did not happen in their territory. And Ukrainian authorities said their primary radar systems had been shut down at the time for scheduled maintenance.

Kiev's explanation "was hardly credible" while Russia's excuse was "bizarre," the relatives shot back.

They are also demanding that the United States hand over its evidence. Only days after the disaster, Washington blamed it on pro-Russian rebels saying they had images showing the missile's trajectory.

Kiev has backed Washington's version of events, while Moscow has long denied any involvement, blaming Ukrainian government forces instead.

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