The crash site of the Malaysia Airlines flight in eastern Ukraine holds many important clues about what happened to the plane. But that site is under the control of pro-Russian separatists who are suspected of involvement in shooting the plane down.
The rebel fighters say they are giving access to investigators, including those from the Ukrainian government, though one Ukrainian official who visited the scene Friday said he was not given full access.
Here are some of the key questions on the investigation into Flight MH17:
The U.N. Security Council called for a "thorough and independent investigation" of the downing of this plane. Is that possible, in the middle of a war zone?
It's likely to be very difficult. Even though there's been talk of a cease-fire to facilitate an investigation, there were reports of fighting Friday within earshot of the wreckage site.
Normally, a crash like this would be investigated by the country in whose airspace it took place.
Pro-Russian paramilitary groups have declared independence in the region, and they've been claiming the airspace, too.
The separatists said today that they would allow investigators access to the site, including international experts.
A team of international monitors did visit the site today. Any word from them?
About 30 observers and experts from the Organization for Security and Cooperation in Europe visited the scene.
The regional governor, who is on the side of the Ukrainian government, charged that the paramilitaries were interfering with the investigation and not giving the experts full access to the wreckage.
What would the investigators be looking for?
Some outside analysts have said the experts may be able to figure out where the plane was hit and determine the missile's trajectory.
That could show where the missile was fired from.
They'll also be looking for fragments of the missile itself, because these weapons generally have markings that show where and when they were made.
One big problem is that the plane was apparently hit at high altitude and the debris and bodies of passengers came down over a very wide area.
Videos show that the site is not very secure — there have been armed men and volunteers tramping around and going through the wreckage, so no matter what the outcome of the investigation might be, there will probably be allegations of evidence-tampering.
There's been a lot of attention to the missile that supposedly brought this plane down. What do we know?
Ukrainian officials say the missile was a Soviet and Russian type known as the SA-11 Buk missile system.
A lot of the discussion has focused on where the paramilitaries would have gotten these weapons and how they would have been able to operate them well enough to hit a fast-moving target at 33,000 feet.
The Ukrainian ambassador to the U.N. said that Ukrainian intelligence has videos and photos showing the rebels have at least two of these systems.
The systems are very complex to operate. It takes a lot of training to use one effectively, so Ukrainian officials are also suggesting that Russian soldiers may have been assisting or manning the weapon.
There was a lot of contradictory talk about the black boxes, the flight data and voice recorders from the plane. Do we know now who has them?
It's unclear now who has them. But the one piece of news that came out was that Russia says it doesn't intend to take custody of the boxes and that they should be handed over to international investigators.
That could be a way to pre-empt claims from Ukraine that Russia somehow manipulated the data to protect the paramilitaries.