KHARKIV, Ukraine (AP) — Two military aircraft carrying the first bodies of victims of the Malaysia Airlines crash left the embattled plains of eastern Ukraine Wednesday, and pro-Russian rebels shot down two Ukrainian fighter jets as fighting flared in the region.

British investigators began work on a pair of "black boxes" to retrieve data on the flight's last minutes, while Dutch officials said they have taken charge of the stalled investigation of the airline disaster and pleaded for unhindered access to the wreckage.

The Dutch government declared a day of national mourning as the country prepared for the arrival of the first bodies. The crash on Thursday killed all 298 people — most of them Dutch citizens — aboard Malaysia Airlines Flight 17.

The two military transport planes, one Dutch and one Australian, departed Ukraine at midday, heading for Eindhoven air base where the flights will be met by Dutch King Willem-Alexander, Queen Maxima, Prime Minister Mark Rutte and hundreds of relatives.

Dutch government spokesman Lodewijk Hekking said the planes carried 40 coffins in all.

"If I have to wait five months for identification, I can do it," said Silene Fredriksz-Hoogzand, whose son, Bryce, and his girlfriend Daisy Oehlers died in the crash. "Waiting while the bodies were in the field and in the train was a nightmare."

Ukraine and western nations are pressing the pro-Russian rebels who control the crash site to allow an unfettered an investigation, something Russian President Vladimir Putin said he would use his influence to achieve. Though confident that a missile brought down the passenger jet, U.S. officials say Russia's role remains unclear.

Ukraine's defense ministry said two fighter planes were shot down about 30 kilometers south of the site of the Malaysia Airlines wreckage. The separatist Donetsk People's Republic said in a statement on its website that one of the pilots was killed and another was being sought by rebel fighters.

While the insurgents deny having missiles capable of hitting a jetliner at cruising altitude, rebel leader Alexander Borodai has said that separatist fighters do have Strela-10M ground-to-air missiles which are capable hitting a target up to an altitude of 3,500 meters (11,500 feet).

Rebel leader Pavel Gubarev wrote on his Facebook page that his men retreated Wednesday from the villages of Chervona Zorya and Kozhevnya, on the Russian border about 45 kilometers (30 miles) from the scene of the crash. Gubarev said 30 rebels had been injured.

Britain's Air Accidents Investigation Branch said Wednesday that Dutch authorities had delivered the plane's voice and data recorders to the agency's base at Farnborough, southern England, where information will be downloaded. Experts will also check for signs of tampering.

The Dutch Safety Board, which is leading an international team of 24 investigators, and said unhindered access to the crash site is critical.

Tjibbe Joustra of the Dutch Safety Board told the AP in a telephone interview that around 25 investigators already are in Kiev analyzing information including photos, satellite images and radar information, but have not yet gained access to the crash site.

"We haven't yet gotten guarantees about security for our way of working. If we go we have to be able to move freely," he said. "We hope to be able to get to the site soon."

Independent military analysts said Wednesday that the size, spread, shape and number of shrapnel impacts visible in an Associated Press photograph of a piece of the wreckage all point to a missile system like the SA-11 Buk.

U.S. analysts have also concluded that an SA-11 was the likely weapon.

Konrad Muzyka, Europe and CIS armed forces analyst at IHS Jane's, said the high number of shrapnel holes in the debris meant that only a fragmentary warhead like the SA-11 could have been used. "The Buk has a 70-kilogram (155-pound) warhead which explodes and sends shrapnel out," he said. The fact the shrapnel holes are folded inwards confirmed that the explosion came from outside the plane, he added.

Justin Bronk, military sciences research analyst at the Royal United Services Institute, said "the size of shrapnel holes is fairly broad, in keeping with what you would expect from a large missile like the SA-11."

The European Union on Tuesday imposed sanctions against more Russian individuals but refrained from targeting entire sectors of the Russian economy while waiting for clearer evidence of Moscow's role in the disaster.

Senior U.S. intelligence officials said Tuesday that Russia was responsible for "creating the conditions" that led crash, but they offered no evidence of direct Russian government involvement.

The officials, who briefed reporters Tuesday under ground rules that their names not be used, said the plane was likely shot down by an SA-11 surface-to-air missile fired by Russian-backed separatists. They cited intercepts, satellite photos and social media postings by separatists, some of which have been authenticated by U.S. experts.

The intelligence officials were cautious in their assessment, noting that while the Russians have been arming separatists in eastern Ukraine, the U.S. had no direct evidence that the missile used to shoot down the passenger jet came from Russia.