ROTTERDAM, Netherlands (AP) — The Dutch prosecutor leading an international criminal investigation into the downing last year of Malaysia Airlines Flight 17 over Ukraine's battlefields said Tuesday the probe has not yet identified specific suspects for possible prosecution, but that he is optimistic it will ultimately be a success.

Fred Westerbeke said the international team has a number of "persons of interest" but declined to identify them. He added that the probe will likely take until at least the end of the year.

The flight heading from Amsterdam to Kuala Lumpur was brought down July 17 over eastern Ukraine, killing all 298 people on board.

The five-nation team involved in the criminal investigation has said that the most likely scenario was that a surface-to-air missile downed the aircraft. Both pro-Russian rebels and Ukrainian forces who were fighting in the region at the time have denied involvement.

Earlier this year, prosecutors appealed for witnesses who could "tell more about the transport, crew and firing of a Buk missile system in the Donbass region" at the time of the downing of the aircraft.

The joint investigation team, made up of experts from the Netherlands, Australia, Malaysia, Ukraine and Belgium, has interviewed more than 100 witnesses so far, Westerbeke told reporters in Rotterdam.

He said that the Boeing 777 being shot down by a Buk missile system remains "the most likely scenario," but added that the investigators have not yet ruled out all other possible causes.

Some relatives of victims of the crash have expressed anger at the slow pace of investigations into the crash.

Westerbeke said he understood frustrations but defended the pace of the probe, saying its results must be "transparent, robust and reach the required evidentiary thresholds in order to stand up in a lawsuit."

A U.N. diplomat said last week that the five countries involved in the investigation want the Security Council to establish a U.N. tribunal that would prosecute those responsible. Last week, their legal experts came to New York to meet with council legal advisers, the diplomat said on condition of anonymity because the discussions have been private.

Westerbeke said he had no preference for where suspects his investigation identifies are brought to justice.

"My greatest preference is a conviction with broad international support," he said. "If that is a U.N. tribunal, in the Netherlands or elsewhere is not so important to me."