WASHINGTON - Major Nidal Malik Hasan, a US army psychiatrist who is suspected of killing 13 people and wounding 30 at a major army base in Texas Thursday afternoon, is in a critical condition after being shot four times by a civilian policewoman. Muslims across the United States braced for a backlash after Arab-American officer, upset over his deployment to Afghanistan, was identified as the shooter at Fort Hood base. The Arab-American Institute said it received one threatening call from an unidentified male shortly after Hasans name was released. The group, which condemned the massacre, said it was expecting more. The Council on American-Islamic Relations (CAIR) held a Capitol Hill news conference on Thursday night to urge calm. No political or religious ideology could ever justify or excuse such wanton and indiscriminate violence, the group said in a statement. Unfortunately, based on past experience, we urge American Muslims, and those who may be perceived to be Muslim, to take appropriate precautions to protect themselves, their families and their religious institutions from possible backlash, the statement added. Lt-Gen Robert Cone, the base commander, said that earlier reports that Maj Hasan had been killed by police were inaccurate. The female police officer who shot Maj Hasan four times, was also in hospital after undergoing surgery for gunshot wounds. Investigation is ongoing but preliminary reports indicate that there was a single shooter, Lt-Gen Cone told a news conference, saying Maj Hasan was in a stable condition. I would say his death is not imminent, Lt-Gen Cone said. Asked if the shootings were a terrorist act, Lt-Gen Cone said, I couldnt rule that out but ... the evidence does not suggest that. Maj Hasan, 39, reportedly armed with a semi-automatic weapon and possibly another handgun, allegedly walked into the Soldiers Readiness Processing Centre and opened fire on fellow soldiers. Maj Hasan, a Muslim of Palestinian roots who was born in Virginia, was understood to have been fighting deployment orders to be sent to Afghanistan later this year. The shooting started at the deployment centre at about 1.30pm local time. Initially, two other soldiers were also detained and questioned for several hours as suspected accomplices, but were later ruled out and released. President Barack Obama, speaking in Washington, said it was a 'horrific outburst of violence. He said: These are men and women who have made the selfless and courageous decision to risk, and at times, give their lives to protect us. Its difficult enough when we lose these brave men and women overseas. It is horrifying when we lose them on American soil. Fort Hood is one of the USs premier installations for training heavy forces and tens of thousands of soldiers go through every year, many heading to Afghanistan. It covers an area of 340 square miles near Killeen, some 50 miles from Waco, Texas. There are up to 65,000 people, including families, on the site at any one time. Maj Hasan, who counselled soldiers returning from the wars in Iraq and Afghanistan, was moved to Fort Hood from the Walter Reed medical complex near Washington earlier this year. A devout Muslim, Maj Hasan prayed regularly at the Muslim Community Center in Silver Spring, Virginia. Despite asking to be discharged from the US Army, he was on the eve of his first deployment to war. In an interview with The Washington Post, his aunt, Noel Hasan, said he had endured name-calling and harassment about his Muslim faith for years after the Sept 11, 2001, attacks and had sought for several years to be discharged from the military. I know what that is like, she was quoted as saying. Some people can take it, and some cannot. He had listened to all of that, and he wanted out of the military, and they would not let him leave even after he offered to repay for his medical training. Hasan spent nearly all of his professional life at Walter Reed Army Medical Center near Washington, caring for the victims of trauma, yet he spoke openly of his deep opposition to the wars in Iraq and Afghanistan. He had been affected by the physical and mental injuries he saw while working as a psychiatrist at Walter Reed for nearly eight years, according to his aunt. He must have snapped, Noel Hasan said. They ignored him. It was not hard to know when he was upset. He was not a fighter, even as a child and young man. But when he became upset, his face turns red. She said Hasan had consulted with an attorney about getting out of the service. At the Muslim Community Center, Hasan stood out because he would sometimes show up in Army fatigues, said Faizul Khan, the former imam there. He was a very quiet and private person. I cant say that people knew him very well other than attending prayers, said Arshad Qureshi, chairman of the board of trustees at the Muslim Community Center of Silver Spring, Virginia. You didnt see him attend anything - school for children or celebrations. He did not go out of the way to engage people. We have thousands of people who come through to pray; he was just one of them.