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WASHINGTON (AP) - An Afghan soldier shot to death a 22-year-old Marine at an outpost in southwestern Afghanistan last month in a previously undisclosed case of apparent Afghan treachery that marked at least the seventh killing of an American military member by his supposed ally in the past six weeks, Marine officials said.

Lance Cpl. Edward J. Dycus of Greenville, Miss., was shot in the back of the head on Feb. 1 while standing guard at an Afghan-U.S. base in the Marja district of Helmand province. The exact circumstances have not been disclosed, but the Dycus family has been notified that he was killed by an Afghan soldier. Marine officials discussed the matter on condition of anonymity because it is still under investigation.

When the Pentagon announced Dycus' death the day after the shooting, it said he died "while conducting combat operations" in Helmand. It made no mention of treachery, which has become a growing problem for U.S. and allied forces as they work closely with Afghan forces to wind down the war.

The Associated Press inquired about the Dycus case after Maj. Gen. John Toolan, the top Marine commander in Afghanistan at the time, said in an AP interview March 7 that the Afghan government has been embarrassed by recent cases of Afghan soldiers turning their guns on their supposed partners.

"I had one just a month ago where a lance corporal was killed, shot in the back of the head, and the Afghan minister of defense was here the next day" to discuss custody of the shooter, Toolan said, speaking from his Regional Command-Southwest headquarters at Camp Leatherneck.

After a negotiation aimed at ensuring the Afghan suspect is prosecuted, the Americans turned him over to Afghan government custody, another official said.

Toolan did not further identify the victim. He mentioned the case while explaining the importance of stopping Afghan treachery as U.S. forces step back from a direct combat role in Helmand and other areas of Afghanistan to a new mission of advising and assisting Afghan soldiers and police. That role, which is in full swing in Helmand, puts U.S. and other NATO troops in closer contact with Afghans at a time when tensions between the two sides have been heightened by an American soldier's alleged killing Sunday of 16 Afghan civilians.

"The Marines and soldiers that are doing the advising work out here understand that if they can't live side by side and operate day in and day out with the Afghans, then they are not going to be able to achieve what they need to achieve as far as relationship building," Toolan said.

A central premise of the war strategy is that success cannot be achieved until Afghan forces are capable of providing security largely on their own and that this will not happen unless American and other coalition forces partner with Afghans at every level to train, advise and mentor them.

In the latest setback, an Afghan civilian interpreter at a British base in Helmand province stole a coalition pickup truck, drove it at high speed onto an airfield ramp and crashed it just as a plane carrying Defense Secretary Leon Panetta was landing Wednesday.

Lt. Gen. Curtis Scaparotti, the No. 2 overall commander in Afghanistan, told reporters that the truck was headed toward a group of U.S. Marines assembled on the tarmac for Panetta's arrival. Neither the Marines nor others in Panetta's welcoming party were injured; the Afghan died of burns sustained in the crash.

Dycus was assigned to 2nd battalion, 9th Marine regiment, 2nd Marine Division from Camp Lejeune, N.C.

Known to friends and family as "Eddie," he graduated from Riverside High School in Greenville in 2008. According to a Mississippi state Senate resolution honoring his life and service, Dycus deployed to Afghanistan on his 22nd birthday, Dec. 12, 2011.

Dycus' killing happened nearly three weeks before the burning of Muslim holy books at Bagram air base, an event that American officials said was accidental but that triggered a wave of protests across Afghanistan and is linked to six other killings of American troops by Afghans.

Two U.S. soldiers were gunned down by an Afghan soldier Feb 23 in Nangahar province; an Air Force lieutenant colonel and an Army major were killed inside the Afghan government office in Kabul and two Army paratroopers were killed by Afghan soldiers in Kandahar province on March 1.

In none of those cases did the Pentagon's casualty announcement mention that the Americans were killed by their supposed Afghan allies. It said, for example, that the two killed Feb. 23 died of "wounds suffered when their unit came under small arms fire." It happened amid an anti-American protest outside the Americans' base. Two protesters were killed by Afghan police there before the Afghan soldier turned his gun on U.S. troops.

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