(CNN) -- A shootout near the U.S.-Mexico border between rival groups with ties to organized crime left 21 people dead Thursday, Mexican police officials said.
Two groups authorities believe were involved in drug and human trafficking clashed in a deserted area about 12 miles south of the border, Sonora State Police spokesman Jose Larrinaga Talamantes told CNN.
Investigators recovered eight bullet-riddled vehicles and seven weapons from the area, about 20 miles south of Nogales, Arizona, he said. The cause of the shootout is unknown, he said, but authorities hope to get more information from suspects in custody.
Police arrested nine people, Larrinaga said. Six of them were treated for nonlife-threatening injuries at a hospital in the state capital of Hermosillo.
The Mexican Army, state police and federal police responded to the area after the shootout, which witnesses said happened about 4 a.m. Thursday (7 a.m. ET). Investigators were still searching for others involved later Thursday, Larrinaga said.
The Mexican federal government said in April that 22,700 people have died in the country since President Felipe Calderon declared war on the cartels shortly after taking office in December 2006.
CNN's Angeles Font and Gustavo Valdes contributed to this report.