President Barack Obama has urged Americans not to jump to conclusions on the motive behind the mass shooting at Fort Hood army base in Texas. In a special news conference, he said: "We don't know all the answers yet and I would caution against jumping to conclusions until we have all the facts." But it is believed the Texas army camp gunman is being investigated for possible links to an extremist website. Military psychiatrist Major Nidal Malik Hasan is in hospital under armed guard after opening fire at a camp used to get soldiers ready for deployment overseas. Within minutes, at least 13 soldiers were dead or dying in the worst mass shooting ever at a US military base. At least 31 others, including two civilians, were injured in an attack described by President Barack Obama as an "horrific outburst of violence". Hasan had come to the attention of law enforcement officials at least six months ago because of internet postings that compared suicide bombers to soldiers who throw themselves on a grenade to save their comrades' lives.