Hillary Clinton's attorney David Kendall obtained top secret clearance from the State Department, he wrote in a letter to Congress this week, in order to help the former secretary of state prepare to testify before Congress about the Sept. 11, 2012 attack on an American compound in Benghazi, Libya.

Since lawmakers and investigators began probing Clinton's use of a personal email server during her tenure of secretary of state, questions have risen about whether the use of the server put sensitive information at risk and who had access to it. Clinton has turned over approximately 30,000 emails to the State Department for release, and inspectors have so far found a few emails with classified information. However, the Clinton campaign maintains the emails were marked classified retroactively.

Obtaining top secret clearance "was unrelated to the 30,490 e-mails provided to the State Department on December 5, 2014; none of those e-mails was classified," Kendall wrote to Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa. "As I am sure you are aware, it was not until nearly six months later that the first email in this group was retroactively classified as 'Secret.'"

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Kendall wrote that he received top secret clearance in November 2014; additionally, his law partner Katherine Turner obtained top secret clearance from the State Department in December 2014.

Kendall was in possession of a thumb drive that held copies of Clinton's emails, but the attorney said the State Department gave him guidance for keeping the information secure, and "even provided physical equipment necessary to safeguard the thumb drive at issue."

Grassley responded that even with his top secret clearance, Kendall did not have the level of authority necessary to possess the sensitive information in the emails in question.

"The transmission of classified material to an individual unauthorized to possess it is a serious national security risk," he wrote in a letter to Secretary of State John Kerry.