Despite mounting evidence that Russian-backed separatists downed a commercial aircraft over Ukraine last week, Western diplomats and law enforcement officials face significant obstacles prosecuting the culprits, according to current and former U.S. officials.

Senior U.S. intelligence officials said Tuesday that they have not yet identified the separatists who operated the missile system that apparently destroyed Malaysia Airlines Flight 17, presenting investigators with their first and perhaps most daunting challenge.

But in a briefing with reporters, the officials continued to build the Obama administration’s case of Russian complicity in a downing of the jet by separatists. The officials provided intelligence material on the path of the missile, voiceprint analysis allegedly of separatists discussing bringing down a plane, shrapnel markings on the aircraft, and material from social-media sites to back administration claims.

The officials, who spoke on the condition of anonymity to discuss intelligence matters, also identified a Russian military installation as a training hub for the separatists and a conduit for weapons to be sent into eastern Ukraine.

Even if the perpetrators are identified, determining their motive and proving their guilt would pose evidentiary and legal challenges for which there is little precedent, according to experts in international law. Establishing a venue for a trial would almost certainly entail intense diplomatic wrangling. Moreover, as crime scenes go, the area in eastern Ukraine where the plane’s wreckage fell is a prosecutor’s nightmare, with reports of looting and tampering with evidence by separatists.

Images claim show the movements of a surface-to-air missile launcher and build up of Russian military activity across the border.

“This is going to be one of the hardest prosecutions imaginable,” said William W. Burke-White, a law professor at the University of Pennsylvania who served as a senior policy adviser from 2009 to 2011 to then-Secretary of State Hillary Rodham Clinton. “Figuring out who actually launched the missile is going to be near to impossible, and any evidence that was there has probably been destroyed.”

Dutch officials said Monday that the Netherlands would take the lead in investigating the doomed flight, which took off from Amsterdam with 298 people aboard, including 193 Dutch citizens. There were no survivors.

“Once the investigation ascertains who was responsible for the downing of the flight MH17, accountability and justice must be pursued and delivered,” Dutch Foreign Minister Frans Timmermans said Monday in an emotional speech at the United Nations. “We owe it to the victims. We owe it to justice. We owe it to humanity.”

On Tuesday, President Obama visited the Dutch Embassy in Washington to offer condolences and pledge to support the investigation. “We will work with them to make sure their loved ones are recovered and justice is done,” Obama said after he signed a condolence book.

A senior White House official said the contours of any prosecution are still being discussed.

“The precise shape of an international investigation — and what prosecution alternatives might be available — is still coming together,” said the official, who spoke on the condition of anonymity. The official said the United States was encouraged by the fact that Ukraine has decided to let the Netherlands lead the investigation.

The two top lawmakers on the House Foreign Affairs Committee called on the U.N. Security Council and the Obama administration to do more to ensure that international investigators have access to the crime scene.

“The credibility of the investigation into this horrific tragedy already has been severely compromised,” Reps. Edward R. Royce (R-Calif.) and Eliot L. Engel (D-N.Y.) said Tuesday in a letter to Secretary of State John F. Kerry. They called the incident a “crime against humanity.”

Russia’s ambassador to the United Nations pledged Monday that his country would assist the investigation, but Western officials and analysts said they suspect that Moscow will be non­cooperative, if not outright obstructionist.

Russian President Vladi­mir Putin “has an obvious strategy of denial,” Burke-White said. “It could well be that whoever did this has gone back to Russia and is able to lay low and use the Russian government as a shield.”

At first glance, the Netherlands, home to the International Criminal Court, where complex legal crimes have been tried, might seem like an ideal venue.

The ICC, however, is unlikely to have jurisdiction, because Ukraine and Malaysia are not member states of the global tribunal. A Security Council referral to try the case there would require the endorsement of Russia, which holds a veto.

The Netherlands may seek to try the case in its own courts, based on the number of its citizens killed, but Russia or other nations could contest its jurisdiction, unless prosecutors are able to prove that the victims were targeted because of their citizenship.

It is also possible the United States could bring a criminal case, because one of the victims was an American citizen.

A prosecution in this case is almost certain to be preceded by intense diplomatic negotiation, said Phillip B.J. Reid, a retired senior FBI agent who was among the lead investigators of the bombing of Pan Am Flight 103, which was downed over Lockerbie, Scotland, in 1988.

“It’s going to have to be a compromise, some sort of agreement that takes place if it’s going to happen,” Reid said.

At the main crash site Tuesday, only two guards watched over a vast area, and they did nothing to stop onlookers from walking through wheat fields that contain the charred and twisted wreckage of the plane.

In nearby villages, potentially important scraps of fuselage that appeared to be pitted with shrapnel marks — potential evidence of an anti­aircraft missile strike — lay along the side of the road. International observers have said that local workers used power tools to slice away portions of the cockpit and first-class cabins.

Reid said that any forensic evidence from the crash site in Ukraine that would be valuable to investigators has probably vanished.

“That crime scene has been contaminated in the worst way,” he said. “This is a war-torn area. There is no law.”

Michael Birnbaum in Hrabove, Ukraine, and Greg Miller and Karen DeYoung in Washington contributed to this report.