FORT HOOD, Texas (CNN) -- Investigators probing a massacre at the world's largest military base pieced through the gruesome scene Friday morning and raided the home of the alleged gunman, searching for answers.

Military officials at Fort Hood in Texas also faced another, equally difficult task: notifying the families of the 13 people killed Thursday inside a military processing center.

"Today on Fort Hood we will observe a day of mourning, remembering in our thoughts and prayers the victims of this incident," said Col. John Rossi, deputy commanding general at the nation's largest military base.

The alleged shooter, Maj. Nidal Malik Hasan, a psychiatrist who worked in a hospital at the base, is blamed for the deaths of 12 soldiers and one civilian, military officials said.

Another 30 people were wounded; 28 of them required hospitalization. About half required surgery, and all were in stable condition Friday morning, said Col. Steven Braverman, hospital commander at Fort Hood.

In news interviews Friday, the base's commander, Lt. Gen. Bob Cone, said witnesses have reported that the gunman yelled "Allahu Akbar" -- Arabic for "God is great" -- during the rampage. Cone said investigators had not confirmed that.

The police officer who shot the gunman, ending the massacre, was among the wounded. Officer Kimberly Munley of the Fort Hood Police Department "just happened very fortunately to be very close to the incident scene," Cone told CNN's "American Morning."

She shot the gunman four times and was wounded herself in an exchange of gunfire with him.

"Really a pretty amazing and aggressive performance by this police officer," Cone said.

Exact details of what happened remained unclear. Investigators were looking into whether some soldiers may have been shot accidentally by others trying to shoot the gunman. Investigators are analyzing "all the rounds, the trajectories, all the weapons, all the shots, where they came from," said Rossi. "That will be determined by the investigators."

The central question they're probing is what may have inspired a member of the military to go on a murderous rampage against his comrades -- particularly one trusted with helping others achieve a healthier mental state.

"He took care of soldiers with behavioral health problems and also evaluated people who had disability evaluations," Braverman told reporters Friday morning.

Asked whether Hasan, 39, had seemed adequately prepared for his job, Braverman responded, "We had no indication otherwise."

Hasan had received deployment orders, military officials said. There were conflicting reports as to whether he was to deploy to Iraq or Afghanistan.

Since 2001, Hasan had been telling his family that he wanted to get out of the military but was unsuccessful, said a spokeswoman for his cousin, Nader Hasan. She added that he told his family that he had been taunted after the terrorist attacks of September 11, 2001, the spokeswoman said.

A police SWAT team and FBI agents raided Hasan's apartment early Friday.

CNN obtained surveillance video from a 7-Eleven convenience store in Fort Hood that shows a man -- who according to the store owner is Hasan -- at the cashier's counter about 6:20 a.m. Thursday, about seven hours before the mass shooting. The man is dressed in traditional Arab garb.

Hasan came in for coffee and hash browns most mornings, the store owner said.

The alleged guman's cousin, Nader Hasan, issued a statement late Thursday on behalf of their family, saying they were "shocked" by the shootings. "We are filled with grief for the families of today's victims," the statement says. "Our family loves America. We are proud of our country, and saddened by today's tragedy."