THE HAGUE, Netherlands (AP) — Defense lawyers for a Russian charged with involvement in the downing of Malaysia Airlines Flight 17 over eastern Ukraine in 2014 said Tuesday that the nearly six-year investigation into the deadly incident still has not produced a definitive picture of what happened and further investigations are needed.

The claim came at the trial of three Russians and a Ukrainian charged with crimes including murder for their alleged involvement in what prosecutors say was a missile attack that blew the Amsterdam-to-Kuala Lumpur flight out of the sky, killing all 298 passengers and crew on board on July 17, 2014.

In a move that could significantly delay the trial, Dutch lawyers representing Russian suspect Oleg Pulatov asked for clarifications and translations of evidence already produced by prosecutors and more investigations, including questioning more than 100 witnesses.

Lawyer Sabine ten Doesschate said she and the defense team would have more requests for further investigations in the future after they have been able to travel to Russia to speak with Pulatov. She said lawyers have not been able to visit Pulatov because of coronavirus lockdown restrictions.

After years of investigations by an international team, prosecutors last year named four suspects: Russians Igor Girkin, Sergey Dubinskiy and Pulatov as well as Ukrainian Leonid Kharchenko. None of them have appeared for their trial, which started in March and is still at a preliminary stage.

At trial hearings earlier this month, prosecutors outlined in detail how the international investigation ruled out other theories and concluded that a Buk missile trucked into Ukraine from a Russian military base was used to shoot down the plane.

But another of Pulatov's defense attorneys, Boudewijn van Eijck, accused investigators of tunnel vision in focusing on that theory and not adequately checking out other possible causes, such as the possibility that MH17 was shot down by a Ukrainian fighter jet or another missile.

Presiding Judge Hendrik Steenhuis adjourned the case until Friday, when judges are expected to rule on the defense requests.