Michèle Flournoy, the most-widely rumored candidate to replace Chuck Hagel as the next secretary of defense, has taken herself out of the running for the job, according to sources familiar with the situation. The decision complicates what will be one of the most important personnel decisions of President Barack Obama’s second term.

Flournoy, the co-founder and CEO of the Center for a New American Security, a think tank that has served as a farm league for future Obama administration officials, would have been the first female secretary of defense had she risen to the position. 

But in a letter Tuesday to members of the CNAS board of directors, Flournoy said she would remain in her post at the think tank and asked Obama to take her out of consideration to be the next secretary of defense. Flournoy told the board members that family considerations helped drive her decision.