- Investigators tried the reach the MH17 crash site for a fourth straight day
- 50 experts, with OSCE monitors, have repeatedly been forced to abandon their attempts
- The U.N. has called for a truce between pro-Russian rebels and Ukrainian forces to allow access
- Investigation expert Matthew Greaves says some evidence will have perished
Nearly two weeks after Malaysia Airlines Flight 17 was blown out of the sky by a suspected surface-to-air missile, the Dutch investigators in charge of finding out what happened have yet to lay eyes on the wreckage or the human remains believed to be still left in the enormous debris field.
Their team of 38 Dutch and 12 Australian experts, accompanied by monitors from the Organization for Security and Co-operation in Europe (OSCE), has repeatedly been forced to abandon its attempts to reach the site.
On Wednesday, the OSCE sent out a reconnaissance convoy from Donetsk to find a possible route to reach the site, according to a tweet from its official Twitter account. It marks the fourth straight day it has tried to reach the area.
Pro-Russian rebels control the area of eastern Ukraine where the Boeing 777 crashed on July 17. They gave Malaysian officials the plane's flight data and cockpit voice recorders or "black boxes" on July 22.
The voice recorder could include audio from the cockpit, which would show whether the pilots knew the plane had been hit. The flight data recorder will give investigators information about engine settings, pressurization and electronic communications.
The rebels have also handed over some human remains from the site. As of Monday, 227 coffins had been sent to the Netherlands, where forensic investigators are working to identify victims. Some 298 people died when MH17 was downed but it is unclear how many complete sets of bodies the coffins contain.
So nearly two weeks after the tragedy, what are the investigators hoping to find out from the crash site itself, and how will the passage of time have damaged the evidence?
CNN staff and Matthew Greaves, Head of the Safety and Accident Investigation Centre at Britain's Cranfield University, explain.
What's stopping investigators reaching the site?
On Monday, the Ukrainian military announced an offensive against a number of key towns on the main road towards the MH17 crash site. That fighting seems to be blocking the OSCE-led investigators, CNN's Ivan Watson says.
Ukrainian military spokesmen deny that government forces control the debris field. They say Ukraine is adhering to a 40 kilometer combat exclusion zone around the crash site.
But the main Ukrainian military spokesman Andrey Lisenko has also said that Ukrainian forces have gotten "very close" to the crash zone, and that they are trying to capture it from the pro-Russian rebels, Watson says.
"As soon as they leave, the experts will be able to start working there," Lisenko told reporters Tuesday.
What's being done to gain access?
The United Nations and other countries have repeatedly called for a cease-fire to allow investigators a safe working environment at the crash site
The Malaysian government had struck a deal with rebels to allow unarmed international police officers to guard the site, but the fighting has made that impossible.
The OSCE Special Monitoring Mission to Ukraine said "intensive planning" was under way to reach the crash site "as soon as it is safe to do so."
Spokesman Michael Bociurkiw earlier said the mission would try "every day" to gain access.
Who has had access to the site since the crash?
After initially being quite hostile to OSCE international monitors on the first day after the crash, the pro-Russian rebels made it quite easy for the delegation and journalists to make daily visits to the disaster zone, Ivan Watson said
Eventually, hundreds of Ukrainian emergency workers and coal miners were brought to the scene to collect more than 100 bodies of victims and bring them to a nearby train station for repatriation.
During his visit to the site last Friday, Watson said, the debris field felt abandoned. There were no guards or investigators on the scene. Much of the debris had been removed within the previous 48 hours.
The only people keeping an eye on the disaster zone were residents from nearby villages, who all appeared to have gone back to farming for a living, he said.
The absence of a Ukrainian government or international force to secure the site has raised concerns about tampering or pilfering.
"It is one of the biggest open crime scenes in the world as we speak, and it is not secured. There's no security perimeter around the 30- or 35-square-kilometer site," Bociurkiw said this week.
What would the process usually be -- if MH17 had not crashed in a war zone?
"You would normally get an instant response from fire and emergency services and maybe coroners and pathologists," said Matthew Greaves, head of the Cranfield Safety and Accident Investigation Centre.
"Normally you'd see that emergency services would be clearing away the bodies as quickly as possible, investigators would not normally get too involved in that. There are two very separate roles there."
Investigators would ideally mark out the wreckage, examine it at the crash site and then take it to a secure location and examine it far more carefully.
Best practice guidelines for crash investigations are set by the International Civil Aviation Organization (ICAO), a branch of the United Nations, Greaves said.
Those rules include who can be involved in an investigation based on the state where the accident occurred, the manufacturers of the plane and its engine, the state where the aircraft was registered and its operator based -- as well as the countries of the passengers on board.
"All of those people play into the investigation because they all have something to offer," he said.
"If an accident had happened in a field in the UK it would be absolutely clear cut who would investigate," he said. "A lot of agencies won't send investigators into an active war zone."
What evidence will investigators be looking for at the site itself?
In a statement Monday, the head of the Dutch-led repatriation team said the team's biggest priority was searching for victims.
If remains were found, they would be recovered immediately, he said: "Their motivation comes from the deep conviction that the relatives are entitled to have their loved ones and their personal effects returned to them."
Matthew Greaves said there could be information in the remains of the victims that could prove useful to investigators but that they would likely obtain this through the work of pathologists.
If shrapnel was found to be embedded in their bodies, it could support the missile theory for example, Greaves said, if the missile was of a type that blew up next to an aircraft.
"I think that probably the aircraft wreckage would tell them far more about that than the bodies," he said.
Investigators try to avoid relying on a single piece of evidence, Greaves said.
"They want, essentially, all of the evidence to support their theory. They won't just ignore something that doesn't seem to fit. They would be constantly looking to corroborate their evidence."
Aren't the data recorders enough?
Just how useful flight data recorders are depends on the accident, Greaves said.
"If you take Air France Flight 447, without the flight recorders it would have been very hard to know what went on," he said. Investigators then had already established that the plane hit the water belly down and they needed to understand why, he said.
"In this case flight data recorders may tell you very, very little. It's possible that this may be a normal fight that just stops," Greaves said. "That would tell you that there was a massive disruption when everything else appeared to be normal.
"They can provide a wonderful starting point even if there's nothing on them that itself is of interest."
Equally when Pan Am Flight 103 was bombed over Lockerbie, the black boxes recorded a snippet of the sound of the explosion, he said.
In the case of MH17, if the information has actually been downloaded off the black boxes, the investigators might be going onto the site with knowledge of some of the flight data, he said.
It's been almost two weeks -- how will the delay have hindered the investigation?
Investigators would be desperate to get to the site itself to "read" the wreckage, Greaves said, as normally the faster investigators arrived the better.
"There's what we call 'perishable evidence' and that's what you'd be looking to gather first," Greaves said.
"If you turn up to an accident and there is a very strong smell of aviation fuel then you'd start off with the idea that it hadn't run out fuel.
"Witnesses recollections are perishable as well. Clearly two weeks after there are going to be some question about how good people's recollection is."
What about the reports of looting?
If the wreckage had been dragged around or there had been wholesale looting it was to the investigators' advantage to know about it in advance, Greaves said.
Normally, Greaves said, investigators would look to ensure they had all the significant plane parts, it might be significant for example if a plane's tail was at a distance from other debris.
"In this example you'd have to keep in the back of your mind that maybe it's been moved or taken," he said.
Will investigators remove wreckage for examination?
In the case of the Lockerbie bombing, which killed 270 people, investigators took every piece of evidence to a hangar, Greaves said.
"In an ideal world you do exactly the same here." But he said given the security situation and the size and weight of the evidence the chances of removing it all were slim.
"They're not going to have days and days and days to slowly pick this up in absolute safety," he said. "They'll be realistic about what they can take."
If investigators could be certain for example that one of the engines had been producing power [a Boeing 777 can fly on just one], Greaves said, they could discount taking those back. Some of the more intact parts of the plane might also be put to one side as normal break-up damage, he said.
Alternatively if there were bits of wreckage that an investigator thought fitted their idea of how shrapnel damage looked, they might want to take them to an expert for examination.
Equally they might take samples if an explosives expert had suggested they look for residue, he said.
Have there been similar situations elsewhere -- what is the precedent for this?
Greaves said he was unaware of a similar situation for an investigation into the downing of a commercial flight in a warzone and that not all countries investigated the shooting down of their military aircraft.
However, he said when a British RAF Nimrod aircraft crashed in Afghanistan in 2006, with the loss of 14 lives, it took accident investigators a few days to get to the site. During that time the unit trying to keep the site secure had to withdraw due to fighting and some of the wreckage was taken away.
"So actually they had to do that investigation with whatever information they could find." The investigators had to refer to photographs taken when the site was first secured and eyewitness accounts from those aboard another aircraft who had seen the plane go up in flames, he said.
It would have been easy to speculate in that instance that the plane had been shot down but the fault turned out to be a fuel leak and those findings prompted an overhaul of the whole military's airworthiness chain, he said.
The same year the British military had to protect investigators after a helicopter was shot down in Basra, Greaves said.
"So sometimes people are put in harm's way to investigate."
Given how much things have been delayed, how long is it likely to be before the families of MH17 victims get answers?
Greaves said it was hard to predict how long the investigators would take to release their findings.
"There is evidence from a number of sources and I'm sure there's evidence from sources that we are not seeing. It's an area of military interest and there are capabilities they have they won't even talk about but might be willing to feed into an investigation," he said.
With the resources potentially available to them, Greaves said the investigators might not be starting cold, meaning they could release a preliminary report relatively quickly. Rather than full analysis and recommendations, this could outline the facts that have been established so far, he said.
Such a preliminary report was issued within two weeks of last November's helicopter crash in Glasgow, Scotland, he said.
Greaves said a full report into a big accident/incident would generally take one to two years.
"They go to great lengths to make sure that they've properly understood the situation. Equally I could imagine this report -- if it's as clear cut as we're being led to believe at moment -- could be two to three months."
If the evidence pointed to a missile, or a criminal act, in theory the investigators could step back at that point, he said. In usual circumstances, they would then let police take over the investigation.
But with MH17 he said a report was likely. "I would be very surprised if they didn't present something."