There are growing concerns that Hillary Clinton's decision to use a private email server to conduct State Department business may have allowed foreign spies to access her correspondence, according to the chairmen of three major Senate national security oversight committees.

Clinton's personal email could have become "a priority target for foreign intelligence services," according to a letter sent to the State Department inspector general by Senate Foreign Relations Committee Chairman Bob Corker (R., Tenn.), Senate Select Committee on Intelligence Chairman Richard Burr (R., N.C.), and Senate Homeland Security and Government Affairs Committee Chairman Ron Johnson (R., Wis.) on March 12.

"We write to you today concerning the recent revelations that former Secretary of State Hillary Clinton and other top aides used non-State Department email addresses and servers to conduct official U.S. Government business," the senators wrote. "We are concerned that diplomatically sensitive, and possibly classified, information may have been transmitted and stored in an insecure manner."

The senators noted that overseas intelligence agencies "continuously probe our government's information systems for weaknesses and attempt targeted intrusions."

"The use of privately maintained information systems that are not protected by federal government experts and key technical capabilities raises serious concerns as those networks may be less secure," they wrote.

The senators asked the State Department inspector general to provide the names of any other officials who worked under Clinton who used personal email for government correspondence, details about the security of Clinton's private email, an assessment on whether any official emails were deleted by Clinton or her staff, and information on whether Clinton or others withheld any emails that should be publicly available.

"We ask that this be an unclassified report, and that a classified annex be provided if necessary," said the letter.

The senators warned that "if a non-government server was known to be a repository for the secretary's emails, it would almost certainly become—if it is not already—a priority target for foreign intelligence services and others."