Kenya's Westgate shopping mall siege: We hear from eyewitnesses and survivors of the attack

The siege in Kenya's Westgate shopping mall appears to be entering the final phase, with a military operation under way aimed at ending the standoff between Kenyan security forces and the al-Shabab militants who stormed it. Who are al-Shabab? Could more have been done to prevent this attack? We hear from eyewitnesses and survivors of the attack.