Fort Hood Army Base in the U.S. state of Texas reopened Thursday evening, several hours after a shooting attack left 12 people dead and 31 wounded, the U.S. department of defense said.

[Chinadaily.com.cn via agencies]

U.S. President Barack Obama lamented the "horrific" incident, calling it a "horrific outburst of violence" and said his thoughts and prayers were with the victims and their families.

"These are men and women who have made the selfless and courageous decision to risk and at times give their lives to protect the rest of us on a daily basis," Obama said when he delivered speech at the closing ceremony of the tribal nations conference in Washington D.C.

"It's difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an Army base on American soil," he said.

Obama said the White House is working with Pentagon, the Federal Bureau of Investigation (FBI) and the Department of Homeland Security to make sure that Fort Hood is secure and the community there has the full resources of the federal government.

"I hope in the meantime that all of you recognize the scope of this tragedy, and keep everybody in their thoughts and prayers," said the president.

Christopher Hogue, U.S. Army spokesman at Fort Hood, told reporters that the shooting took place 1:30 p.m. CST (1930 GMT) at a family readiness area at the base. The shooter soldier was included in the number of death and another two soldier suspects have been apprehended.

The attacker who was killed after opening fire at the base was reported to be Maj. Nidal Malikl Hasan, a psychiatrist in his 30s who was going to be dispatched to Iraq. Hasan "was disturbed" about his deployment, Senator Kay Bailey Hutchison told a local news organization.

The Army closed the base covering 339 square miles, the largest active duty armoured post in the U.S. with about 40,000 troops. A message on the Fort Hood Web site said, "Effective immediately, Fort Hood is closed." "This is not a drill. It is an emergency situation."

"Tons of sirens" wailed at the base after the shooting, witnesses said. The basewide siren and announcement system instructed residents to seek shelter, lock their doors and turn off the air conditioning. Some residents said they thought the sirens could be a training exercise.

Nine victims in the shooting were taken to Scott & White Memorial Hospital, the hospital said on its website. All the victims are suffering from wounds and all are adults, according to the hospital. The hospital on its website also called for donation of blood of all types.

National officials offered condolences and support to the victims and families.

"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, " Defense Secretary Robert Gates said in a statement.

Vice President Joe Biden expressed sympathies for the families of the victims.

"We are all praying for those who were wounded and hoping for their full and speedy recovery," he said in a statement. "Our thoughts and prayers are also with the entire Fort Hood community as they deal with this senseless tragedy."

"I was saddened to learn of the tragic incident at Fort Hood. Laura and I are keeping the victims and their families in our thoughts and prayers during this difficult time," former president George W. Bush said in a statement issued from his office in Dallas, Texas.

Texas governor Rick Perry said the appropriate authorities, including the FBI, other federal authorities and the military, are handling the situation right now.

In honor of those killed in the shooting attack, Perry said he's ordered flags to be flown at half staff in Texas until Sunday.