Defense lawyers for a Russian charged with involvement in the downing of Malaysia Airlines Flight 17 over eastern Ukraine in 2014 cast doubt Monday on prosecutors’ assertions that the passenger jet was shot down by a Buk surface-to-air missile.

The comments came as Dutch defense lawyers for Oleg Pulatov began listing their requests for further investigations in the international probe into the July 17, 2014, downing of MH17.

Defense lawyer Boudewijn van Eijck pointed out to judges that prosecutors were unable to seal off or carry out forensic investigations at the crash scene, which was in a region controlled by pro-Russia rebels fighting against Ukraine’s government.

“For that reason it can’t be ruled out that evidence went missing, was manipulated or even was augmented,” Van Eijck said.

The trial of three Russians and a Ukrainian charged with murdering all 298 passengers who died on board the Amsterdam-Kuala Lumpur flight is still at an early phase, when defense attorneys can ask judges to order further investigations.

None of the suspects has appeared for trial. Only one, Pulatov, has lawyers representing him in court. All of the suspects face a maximum sentence of life imprisonment if convicted.

After years of investigations, an international team of investigators and prosecutors last year named four suspects: Russians Igor Girkin, Sergey Dubinskiy and Pulatov as well as Ukrainian Leonid Kharchenko.

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 Van Eijck accused investigators of tunnel vision in focusing on that theory and not adequately checking out other possible causes, such as the possibility MH17 was shot down by a Ukrainian fighter jet.

“It certainly seems that they are extremely attached to the Buk missile scenario,” Van Eijck said.

Another defense lawyer, Sabine ten Doesschate, asked judges to order new investigations including interviews with witnesses who claimed to have seen fighter jets in the sky around the time MH17 was shot down, suggesting that the Ukraine air force could even have used passenger jets as “human shields” for its fighter planes.

She also asked for more investigations into radar images from the day of the downing.

Van Eijck also called into question the international investigation’s reliance on information from Ukraine to build the case against the four suspects, saying that one Ukrainian prosecutor has said he wanted to prove Russia was involved.

“That does not reflect a willingness and a determination to uncover the truth,” Van Eijck said.

(AP)