CIUDAD JUAREZ, Mexico — The bones of nine people were found in desert graves in violence-plagued northern Mexico, raising the number of remains found there since the weekend to 20, authorities said Monday.

The 20 male remains were discovered in 15 graves in the Juarez Valley, near the border city of Ciudad Juarez, anti-organized crime prosecutor Jorge Gonzalez Nicolas told a news conference.

With backing from US authorities, more than 100 police officers, sniffer dogs and forensics experts took part in the search in the valley that surrounds Ciudad Juarez, one of Mexico's most dangerous cities.

Two days after unearthing the bones of 11 people, authorities found nine others on Sunday and Monday in the valley located in Chihuahua state.

After the first bodies were discovered on Saturday, authorities said they appeared to have been shot dead in 2009 and 2010, amid a turf war between the Juarez and Sinaloa drug cartels, which have fought for control of drug smuggling routes to the United States.

More than 60,000 people have died in drug-related violence across Mexico since President Felipe Calderon, whose six-year term ends on Saturday, launched a military offensive against cartels in 2006.