1 5 Spain March 11 Madrid train bombings.[27][28] 192 2050 Saudi Arabia April 21 A car bomb explodes outside a building originally used by the Saudi police.[29] 5 148 Saudi Arabia May 29–30 The 2004 Khobar massacre was a shooting and bombing campaign led by four Al Qaeda-linked militants against two oil industry installations and a residential compound in Al-Khobar, Saudi Arabia.