WASHINGTON -- US Defense Secretary Robert Gates and Chairman of Joint Chiefs of Staff, Admiral Michael Mullen, expressed deep sorrow over the mass shooting at a military base in Texas that left 12 people dead and 31 wounded.

"I am deeply saddened by the tragic events today at Fort Hood. My thoughts and prayers are with the families of the fallen, the wounded, and all those touched by this incident," said Gates.

"I can pledge that the Department of Defense will do everything in its power to help the Fort Hood community get through these difficult times," he added.

Officials at Fort Hood, the largest US military base in the world, said that a soldier opened fire in a busy processing center where troops go before being deployed to Iraq and Afghanistan. Twelve people including the shooter were killed.

The shooter killed by police was identified as Nidal Malik Hasan, a military psychiatrist.

Mullen also said he was "deeply saddened by the news," and added that the Army was investigating what appears to be the deadliest mass shooting at a US military base in modern history.

"It is important we let that work complete before we speculate about the circumstances leading to this senseless violence," he added.