WASHINGTON The families of thousands of dead American soldiers may have been mourning at the wrong graves for years, it was revealed on Thursday, as more than 6,000 headstones at Arlington cemetery may have been mixed up.

There was outrage among families, the American Legion and others last month when the number of graves that had been misnamed was thought to be 211. The higher figure was revealed on Thursday at a Senate investigation into how there could be so much confusion over the headstones at the US national military cemetery.

Senator Claire McCaskill, who is heading the Senate committee conducting the inquiry, said “We now know that the problems with graves at Arlington may be far more extensive than previously acknowledged. At a conservative estimate, 4,900 to 6,600 graves may be unmarked, improperly marked, or mislabelled on the cemetery's maps.”

Arlington, just across the river from Washington, is one of the most revered and emotional sites in the US, having been used for members of the military since the civil war. There are an estimated 300,000 graves, including those of John F Kennedy and his two brothers, Robert and Edward.

Fresh graves are being dug regularly for the dead from Afghanistan and Iraq.

Army investigators found that Arlington has been mismanaged, with remains buried in graves listed as empty and others had headstones but no remains. Others were unmarked or misidentified, or did not appear on cemetery maps.

At the Senate hearing, John Metzler, who was superintendent of Arlington for 19 years, said he accepted “full responsibility” for the problems. He said the paperwork relating to the graves was complex and this gave rise to errors.

“Personally it is very painful for me that our team at Arlington did not perform all aspects of its mission to the high standard required,” he said.

Metzler and his deputy, Thurman Higginbotham, were forced to retire last month after army investigators discovered the initial mix-up over the 211 graves.

Metzler cited a 35 per cent cut in staff and the complexity of burying 6,000 people a year at the cemetery.

“Those staffing losses were to be offset by increased opportunities for outsourcing to private contractors,” Metzler said . “As experience has shown, however, that approach does not always result in the most effective or efficient solution.”

Senators appeared frustrated and exasperated at times with Metzler's explanations for the mix-ups. He said that problems down the years had been quickly remedied and that though maps used by cemetery employees were mislabelled, this did not mean people were necessarily in the wrong graves.

The anger among senators applied across partisan lines. McCaskill, a Democrat, said “The notion that you would come in here and didn't know about it until a month ago is offensive. You did know about it, and you did nothing.”

Scott Brown, the Republican who won Ted Kennedy's Massachusetts seat in January, said “I don't think we're getting straight talk here.”

Higginbotham answered some questions but invoked his right, under the fifth amendment, not to respond to others.

He said “It was always conceptual that anything done by hand for 40-plus years that there would have to be some errors somewhere.”

When the original mix-up was first revealed, the secretary of the army, John McHugh, apologised. At the time, the Stars and Stripes military paper quoted families expressing concern.

Ami Neiberger-Miller, whose brother Christopher Neiberger is buried in Arlington, told the paper “I think anyone would be concerned and alarmed.

“I appreciate that the army came forward with this information and talked about it in such an open way and that the secretary of the army actually apologised to surviving families for this.”

The American Legion expressed disappointment at the time that “our nation's heroes are treated in such an undignified manner”.—Dawn/Guardian News Service