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

BBC News:

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.

Mexico opposition wants army off drug crackdown

Drug war has come under fire

Mexico takes a risk using soldiers in drug war

Powerful and well-armed Mexican cartels fight back against the army's anti-drug siege