Chairman of the Joint Chiefs of Staff Adm. Mike Mullen on Friday tweeted prayers for the victims of the shooting at Ft. Hood, Texas yesterday that killed 13 and wounded 30.

Mullen said he was "deeply saddened" by Maj. Nidal Malik Hasan's "senseless" shooting at the base but refused to speculate about Hasan's motives.

He tweeted last night and retweeted a message from his wife this morning:

Deeply saddened by the shooting @ Fort Hood. My thoughts & prayers w/ wounded, families of those killed, & all @ Ft. Hood.

Ft. Hood details still unclear. Army investigating. Important we not speculate about circumstances leading 2 this senseless violence.

RT @deborahmullen: Difficult 2 express how very sad I am @ t/senseless shooting @ Ft. Hood. I pray 4 those killed/injured-families/friends

Mullen added details about a moment of silence:

Please join OUR military in a moment of silence & prayer 4 the fallen @ Fort Hood - TODAY @ 2:34pmET

Investigators are still determining why Hasan decided to turn a gun on his fellow soldiers. Ft. Hood's base commander Lt. Gen. Robert Cone said there have been unconfirmed reports that Hasan shouted "Allah Akbar!" before he opened fire.

Hasan was set to deploy to Afghanistan. Hasan reportedly made remarks in opposition to the conflicts in Iraq and Afghanistan.