A 25-year-old police officer has been killed after a bomb exploded under his car in Omagh, County Tyrone.The device exploded under the vehicle outside his home in Highfield Close, off the Gortin Road, just before 1600 BST on Saturday. He is the second policeman to be killed since PSNI was formed out of the RUC. Since 2007, dissident republicans have planted dozens of booby-trap bombs under the private cars of police officers. The bombs have failed to detonate, but two policemen lost their legs in attacks in May 2008 and January 2010.