Police and Mexican army troops arrested four members of a criminal assault force that overran a town near the Arizona border and set off gunbattles and executions that left at least 22 dead in violence linked to drug cartels. Luis Pena Molina, town secretary of Banamichi, said Thursday that "helicopters are combing the whole area" looking for the remnants of the estimated 50 gunmen who assaulted the nearby town of Cananea, 30 miles (20 kilometers) south of the U.S. border. He said four gunmen had been detained. The violence began Wednesday, when men armed with assault rifles and riding in 10 to 15 vehicles pulled four lightly armed city police officers out of their cars and executed them in a park. The assailants fled to the nearby hills with authorities in pursuit. The gunmen ditched their vehicles, commandeered horses and forced ranch hands to serve as guides, according to an account from a man abducted by the armed gang.