MH17 victims' families sent a letter to Dutch Prime Minister Mark Rutte, accusing Dutch investigators of a "completely botched" investigation into the crash. Rutte is pictured during a joint news conference with Malaysia's Prime Minister Najib Razak in Putrajaya Nov. 5, 2014. Reuters/Olivia Harris

The families of Malaysia Airlines MH17 crash victims want the United Nations to take over the investigation of the downing of the aircraft, citing frustration over what they say is the failure of Dutch authorities to build a proper case, according to a Reuters report.

Despite launching the largest criminal investigation in their country’s history, the families say Dutch authorities “completely botched” the investigation in a letter signed by 20 relatives of victims and sent to Dutch Prime Minister Mark Rutte on Friday. The Dutch Safety Board took over the investigation within days of the crash, as many of the victims were Dutch. Rutte disagreed with the families' assessment: “We have done everything we could... In view of the safety situation and the weather, we cannot do anything more right now.”

MH17 was shot down and disintegrated in mid-air over territory held by pro-Russian separatists battling the Ukrainian military in eastern Ukraine on July 17. All 298 passengers and crew were killed. The crash spread wreckage across an 8-square-mile area that been fiercely fought over by rebels and the Ukrainian military.

It took days for investigators to reach the wreckage, during which rebels and Ukrainian authorities accused each other of tampering with evidence. Dutch authorities have transported wreckage of the Boeing 777 to the Netherlands to be reassembled. The most recent load was moved this week, but some wreckage remains strewn across fields in eastern Ukraine. Dutch authorities have not come to a conclusion over what brought down the aircraft, and they are still having difficulty reaching the crash site as fighting heats up in eastern Ukraine ahead of a proposed truce.

A crane transports a piece of the Malaysia Airlines flight MH17 wreckage at the site of the plane crash near the village of Hrabove (Grabovo) in Donetsk region, eastern Ukraine November 20, 2014. Reuters/Antonio Bronic

The German mother of one victim filed a lawsuit against Ukraine with the European Court of Human Rights over the tragedy on Sunday, demanding $1 million from the government for negligent homicide, according to AFP. The suit argues that Ukraine is responsible for the tragedy because it didn’t close the airspace over embattled eastern Ukraine.

More than five Ukrainian military aircraft had already been shot down by rebels by the time MH17 was downed by what evidence suggests was a rocket fired by separatists using a sophisticated high-altitude air defense system. Ukraine accused the Russian military of supplying the missile system to separatists, but Moscow denies that allegation.