FORT HOOD, Texas — The U.S. Army psychiatrist accused of opening fire on fellow soldiers at a medical processing center here last week faces charges of premeditated murder for the deaths of a dozen soldiers and one civilian.

Maj. Nidal Malik Hasan could also face additional military charges for the Nov. 5 attack, said Christopher Grey, a spokesman for the Army’s Criminal Investigation Division.

Flanked by an FBI agent, Texas Ranger and an Army CID official leading the investigation, Grey said Thursday that the formal charges against Hasan are “the first step in the court-martial process.”

Officials believe that Hasan was the only gunman “involved in the actual shootings,” Grey told reporters at a news briefing. He had no appointments, orders or other legitimate reasons to be at the Soldier Readiness Processing Center on the afternoon of the attacks. A multi-agency task force continues to follow a complex array of leads and clues about possible motives for the attack.

Authorities and some soldiers and civilians who knew Hasan have previously said that he was upset about being deployed overseas to a battle zone where the U.S. was fighting Muslims. He is a native of the U.S., born to Palestinian immigrants. Though his parents were not observant, friends and family have said that Hasan became devout after his mother died in 2001. Co-workers at Walter Reed Army Medical Center have said that he was aggressively critical of U.S. involvement in Iraq and Afghanistan and insistent that Muslims should be allowed to leave military service as conscientious objectors.

“We’re looking at every reason for this shooting,” Grey said. “We’re aggressively following every possible lead.”

A dozen of the wounded remain at local hospitals in stable condition, another Army official said Wednesday. One of them is still being treated in an intensive care unit.

Grey said investigators have not yet been able to talk to every victim because of the severity of their injuries.

Grey declined to answer questions, telling reporters that authorities do not want to jeopardize the ongoing probe or the legal case.

The processing center where Hasan drew two pistols, one equipped with a laser sight, and began gunning down soldiers is still sealed off as a giant crime scene, Grey said. Investigators have examined more than 100 cars in nearby parking lots for bullet holes and are still combing the warren of offices, cubicles and open areas of the processing center for evidence. Four adjacent buildings and surrounding land are also being probed for clues, and authorities have “no estimated time line for when the crime scene will be released.”

“Please be patient and understand the magnitude and complexity,” Grey said.

The 39-year-old psychiatrist allegedly entered the processing center and began firing on unarmed soldiers who were undergoing medical processing for overseas deployments to Iraq and Afghanistan. In addition to the 12 soldiers and civilian contractor killed, 38 people were wounded. It is the deadliest incident at any U.S. military base.

Two civilian police officers who work at Fort Hood responded to reports of a shooting and confronted Hasan. They shot and wounded him four times, and one of the officers was wounded in the melee.

Hasan is still being treated at a military hospital in San Antonio, where he is being held on what Grey described as “pretrial restriction.” Officials have said that Hasan has refused to speak to investigators. A lawyer retained by Hasan’s family has said he asked that his client not be questioned without a defense attorney present.

Col. John Rossi, a Fort Hood spokesman, told reporters that the base is recovering from what the Army’s top general described as “a kick in the gut.”

“Today, Fort Hood continues to grieve and heal,” he said. “At this time, Fort Hood has gotten its breath back and we continue to move forward.”

Base mental health providers have been joined by at least 100 outside mental health specialists to offer soldiers and their families counseling, stress management and other support services. Rossi said every wounded soldier has already received critical-incident stress debriefings, and the mental health efforts have so far resulted in “3,000 individual contacts.”

“We’re guarding against any premature determination that everything is okay,” he said. “This is not only a medical activity. This is a leadership activity.”

More than $165,000 in donations have come from around the country to support survivors and families of the dead, Rossi said.