A stunned nation battled to understand Friday why a Muslim army psychiatrist may have snapped, mowing down 13 people and wounding 30 others in a massacre at a sprawling US military base. Alleged shooter Major Nidal Malik Hasan, 39, a specialist in combat stress, was under guard and on a ventilator after a police officer shot and seriously wounded him during Thursday's rampage at Fort Hood, Texas, officials said. But so far Hasan, who had been fighting orders to deploy to Afghanistan, has not spoken about his actions. 'We're not going to speculate about motives,' Colonel John Rossi, the deputy Fort Hood commander, told reporters here early Friday, as the US military ordered the nation's bases to tighten security. US President Barack Obama warned against making any hasty assumptions as agents launched an investigation. 'We don't know all of the answers yet. I would caution against jumping to conclusions until we have all of the facts,' Obama said, ordering flags to fly at half-staff at the White House and federal buildings as a sign of mourning. 'What we do know is that their families, friends, and an entire nation is grieving right now for the valued men and women that came under attack.' It remained unclear whether the suspect Hasan had snapped after counseling scores of troops scarred by the wars in Iraq and Afghanistan, or whether he was motivated by other deeper convictions. Witnesses reportedly heard him shout 'Allahu Akbar!' (God is greatest) as he opened fire in a troop processing center with a semiautomatic weapon and a handgun. 'There are first hand accounts here from soldiers here that are similar to that,' base commander Lieutenant General Bob Cone said. Although 'Allahu Akbar' is a Muslim prayer, it has come to be associated with Islamic militants as they carry out attacks or suicide bombings. A surveillance video aired by CNN showed the major buying breakfast wearing traditional Muslim garb at a base store just hours before the shooting. The bloodshed dealt a new blow to US forces already under severe strain from repeated combat tours and plagued by a rise in suicides and depression. Most of the victims were military personnel, and 28 victims were still in hospital Friday. Fort Hood, by area the world's largest US military base, has borne the brunt of the wars in Iraq and Afghanistan. Troops based here have suffered the highest number of casualties and have undertaken multiple tours of duty. Secretary of the Army John McHugh, and Army chief of staff General George Casey were heading to the base Friday to observe a moment's silence and prayer at 1934 GMT. Obama would also attend a memorial service due to be held in the coming days, the White House said. Amid the sorrow, the shooting raised delicate questions about Muslim soldiers serving in the US Army, as some Muslim groups feared a huge backlash. Seeking to allay those concerns, Pentagon spokesman Geoff Morrell said Friday that the army always has 'a deep and enduring concern that everyone be treated with respect.' Hasan was born in the United States to Palestinian parents who had moved from a small town near Jerusalem. His cousin Nader Hasan, writing on behalf of the family as Hasan's parents are dead, said they were stunned by Thursday's events and stressed they all considered themselves Americans. 'Our family loves America. We are proud of our country, and saddened by today's tragedy,' Nader Hasan said in the message posted on The Washington Post website. Nidal Hasan's aunt, Noel Hasan, told the daily her nephew had been subjected to harassment about his faith since the September 11, 2001 attacks and had repeatedly sought to be discharged. An unsigned video praising the attack appeared on a Islamic militant website called the Fallujah forum on Friday, and was swiftly picked up by other sites. The shooter 'seemed upset and did not want to participate in this war, and he did not want to be among the ranks of infidels against his Muslim brothers,' the video said, offering its congratulations. Some were convinced Hasan lost control due to the strains of his job. 'The questions many here in Texas would ask are when will this nightmare end? And how will we properly care for the 'walking wounded' we have created?' said Fran Hanlon, a member of a Fort Hood outreach center, in a statement. 