Mexican police officers murdered

Seven Mexican police officers were assassinated in about an hour's time in what authorities said was a co-ordinated effort that followed months of relative calm in Tijuana, a border city stricken by drug-fuelled violence.

Two other officers and a shop employee were injured.

After four officers were killed by gunfire outside the convenience store, police scanners hummed with "narcocorridos", or drug ballads.

One voice threatened over the airwaves that 30 officers would be killed.