WASHINGTON -- Investigators digging into the scandal at Arlington National Cemetery believe that thousands of graves -- rather than hundreds -- may be unmarked, wrongly identified or mislabeled on maps. 

Documents released this afternoon by the Senate Homeland Security Subcommittee on Contracting Oversight  found that between $5.5 million and $8 million was spent for failed grave-tracking systems over the years.

"Despite these expenditures, Arlington National Cemetary still does not have a system that can accurately track graves and manage burial operations," according to a memo from investigators.

The failures will be the subject of a hearing Thursday conducted by Sen. Claire McCaskill, D-Mo., chair of the contracting subcommittee.

A spokeswoman for McCaskill said this afternoon that subpoenas were issued for two Army officials who were in charge of the cemetery before they retired recently.

The officials had refused an invitation to attend.

In June, the Army's inspector general released a repart detailing major flaws at Arlington, where more than 330,000 people are buried.

The report found hundreds of mistakes associated with graves, including mishandling of cremated remains and urns with remains discovered in the cemetery landfill.