Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, and Gen. David Petraeus, commander of Central Command, have sent letters to Defense Secretary Robert Gates endorsing Gen. Stan McChrystal, NPR's Tom Bowman has learned from someone familiar with the letters.

Tom tells us:

"Gates, meanwhile, is supportive of McChrystal, though is still wary of a large American footprint of troops in Afghanistan.

McChrystal's second report, which includes various options for troop increases, is expected to reach Washington by week's end. Sources say the high end is 40,000 troops, with 60 percent trainers and the rest combat troops."