A Dutch-led probe into the shooting-down of flight MH17 over Ukraine in 2014 released new intercepted phone calls between high-ranking Russian officials and suspects facing trial over the crash.
Investigators said they were making a "new witness appeal" based on "recorded telephone calls between the leaders of the DPR (Donetsk People's Republic, a separatist group) and high-ranking Russian officials."
"Ties between Russian officials and DPR leaders appear to have been much closer" than originally believed, Andy Kraag, the head of Dutch police's Criminal Investigations Division, said in a video statement on Thursday.
Investigators said in June that they were going to put three Russian nationals and one Ukrainian on trial in the Netherlands in March 2020 over the downing of the Malaysia Airlines jet, although they will likely be tried in absentia.
The Boeing passenger plane travelling from Amsterdam to Kuala Lumpur was shot down by a Russian-made BUK missile in 2014 over eastern Ukraine. All 298 people on board were killed.
Investigators said on Thursday they had now established that Russian officials and the Ukrainian rebels used secure communications that "appear to be provided by the Russian security service FSB".
The special phones were "used by Russian top officials in their contact with the fighters", said David Nelson, senior investigations officer with the Australian Federal Police.
"We have released recorded conversations they had about administrative, financial and military business," Nelson added.