Mexico's government has sent federal police re-enforcements to a northern area where 22 people died in a battle between drug gangs and security forces
Heavily armed Gulf Cartel gangsters travelling in a convoy ambushed police in the town of Cananea in Sonora state, bordering the US, on Wednesday.
They killed five policemen, sparking a battle that left 15 gang gunmen and two civilian bystanders dead.
President Felipe Calderon has ordered troops to root out drug traffickers.
This week's violence in Sonora is the bloodiest seen during a five-month offensive.
Up to 50 well-armed gunmen arrived in the town of Cananea on Wednesday, travelling in a fleet of rugged vehicles.
They captured and executed four lightly armed policemen in a public square.
Police teams followed the attackers to their hideout in the hills, killing 15 gunmen.
On Thursday, police helicopters and hundreds of ground forces searched for fleeing gunmen in the Sierra Madre, near Cananea.
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