After Chuck Hagel’s sudden resignation as secretary of defense Monday morning, speculation has moved from the reasons for the de facto firing of the Pentagon chief to who will replace him. Hagel will remain in the position until a successor is found, and confirmation of the nominee most likely will be the job of the incoming Republican-majority Senate.

Pentagon accusations that the White House is micromanaging its every move may complicate the search for a replacement.

It’s unlikely Obama will opt for a recently retired military officer because of a federal law that prohibits recently retired senior general officers taking the position for seven years. This eliminates obvious candidates such as James Mattis and Stanley McChrystal. While there is a possibility of Congress granting a waiver for those generals, it’s unlikely, and leaves the door open for some familiar names connected with the position in the past.

High on the list is Ashton Carter, a former deputy secretary of defense until December 2013. Once described in a CNN article as a man who can “keep the trains running,” Carter’s recent experience at the Pentagon would bring the stability Obama is seeking in the role, but Carter may lack the experience required to deal with the continued terrorist threats in Syria and Iraq, and with the troop withdrawal in Afghanistan.

The defense-oriented academic was undersecretary for acquisition, technology and logistics under Defense Secretary Robert Gates from April 2009 to October 2011, noted for providing troops with vehicles that were better protected from roadside bombs. He left for the deputy secretary of defense role.

Michèle Flournoy, the undersecretary of defense for policy from February 2009 to February 2012, is known as a centrist who can work with both Republicans and Democrats, and has apparently been positioning herself for the top defense role since she left government in 2012. Her time away from the limelight has not been spent entirely away from the world of politics. During her time off, she became a senior adviser on Obama’s last campaign while working at a defense think tank she founded, which means her name may not have been far from Obama’s thinking. If she did take the position, she would be the first female secretary of defense in U.S. history.  

Robert Work is the current deputy defense secretary and may be a logical choice if Obama is looking for someone who intimately knows what’s happening inside the Pentagon right now. On the other hand, however, Obama may be looking to clear the decks and start fresh.  

Sen. Jack Reed, D-R.I., who turned down the opportunity to replace Robert Gates in 2010, reportedly said he’s not interested in the position. Had the Democrats retained their majority in the Senate, conventional wisdom on Capitol Hill and among the defense industry insiders suggested he would take the job of Senate Armed Forces Committee chairmanship, but increasingly that position looks like it will go to Sen. John McCain, R-Ariz. Reed may instead take on the chairmanship of the Senate Banking Committee.  

“Senator Reed loves his job and wants to continue serving the people of Rhode Island in the United States Senate,” said Chip Unruh, Reed’s spokesperson. “He has made it very clear that he does not wish to be considered for secretary of defense or any other Cabinet position. He just asked the people of Rhode Island to hire him for another six-year term and plans to honor that commitment.”