Ex-Pentagon official Ashton Carter may return as No. 1

Obama selects Ashton Carter as next Pentagon chief
Obama selects Ashton Carter as next Pentagon chief   

A senior GOP senator confirmed on Tuesday that President Barack Obama had chosen Ashton Carter, a former Pentagon official, as his nominee to be Defense secretary, according to The Associated Press.

The president will not make an announcement on Tuesday, but officials told NBC he will likely reveal his decision Wednesday or later in the week.

If Carter's nomination is confirmed by Senate, he will succeed Republican Chuck Hagel, who resigned last week after less than two years.

Read MoreInside North Korea's elite cyberwarfare unit

Ashton Carter
Lee Jin-man | AP
Ashton Carter

Carter was deputy secretary from October 2011 to December 2013. His profile was boosted by his contributions in planning the raid that killed Osama bin Laden, AP reports. Under the Clinton administration, Carter was the assistant secretary for international security policy.

Other candidates are also being considered, officials said. The list has narrowed—former Pentagon general counsel Jeh Johnson, currently secretary of Homeland Security, is no longer being considered, administration sources told NBC. Johnson was considered too controversial due to his involvement in Obama's immigration policy. Michele Flournoy, former undersecretary of Defense, requested to be withdrawn from consideration.

Watch More: Behind Hagel's exit from Pentagon

Many experts say the position is difficult to fill due to timing. In the final two years of the Obama administration, the defense secretary must address the growing threat from ISIS in Syria and Iraq and the difficulties associated with pulling out from Afghanistan amid tightening budgets.