Dutch prosecutors say they want to question a "person of interest" in Ukrainian custody who they believe is directly connected to the downing of Malaysia Airlines flight MH17 five years ago over eastern Ukraine.
The individual, Ukrainian national Volodymyr Tsemakh, reportedly oversaw an air-defense unit among Russia-backed separatists in a town near where the jet came down.
He is currently thought to be among the prisoners being discussed in a potential prisoner exchange between Ukraine and Russia.
All 298 people on board were killed when MH17 was shot down by a Russian-made missile from territory held by Russia-backed separatists as it was flying from Amsterdam to Kuala Lampur in July 2014.
"We would like to talk to Mr. Tsemakh and ask him questions, so we would rather have him available for the investigation in Ukraine," Brechtje van de Moosdijk, a spokeswoman for the Dutch-led MH17 investigation, told AFP and AP.
Unconfirmed reports suggest the prisoner swap has stalled because Moscow is demanding Tsemakh's inclusion.
Kyiv is seeking the return of 24 sailors detained by Russia last year off annexed Crimea, as well as filmmaker Oleh Sentsov and others whom rights groups and the government in Kyiv say are "political prisoners" in Russia.
Tsemakh is a Ukrainian citizen.
"He is now in Ukraine, in a Ukrainian prison cell, and if he is being exchanged, of course it's hard to say that we can still question him," Van de Moosdijk said.
An international Dutch-led investigation has already concluded that the commercial airliner was shot down by a Russian-made Buk missile that was fired in territory held by Moscow-backed separatists. Investigators maintain the missile system belonged to a Russian military unit and that it was transported from and back to Russia after being used.
Three Russians and a Ukrainian were indicted over the downing of MH17, and court proceedings in the Netherlands are scheduled for March, but the four suspects are most likely to be tried in absentia.
Russia called the charges against the country's citizens "absolutely unfounded" and said the investigators based their findings on "dubious sources of information," accusing them of rejecting evidence the Kremlin has provided.
Moscow has also aired its own theories on the shoot-down but never provided solid evidence.
Tsemakh is not one of the four indicted.
The Ukrainian Security Service (SBU) apprehended him on June 27 in the Donetsk region city of Snizhne, which is held by Moscow-backed separatists and is 20 kilometers from the Russian border.
According to the Dutch-led investigation, the Buk missile was fired 6 kilometers south of Snizhne.
TV footage obtained by Current Time, the Russian-language network led by RFE/RL in cooperation with VOA, showed Tsemakh claiming that he was in charge of an antiaircraft unit and that he helped hide the missile system in July 2014.
He also shows the interviewer where the civilian airliner fell.