Federal authorities are investigating what caused the explosion , the attorney general 's office said . NEW : Mexico 's labor secretary says Chilean experts will visit the site Wednesday . The U.S. State Department said it was aware of reports that Chile had agreed to send a team to help with rescue efforts in northern Chile. In a Twitter post , he says the mine had 25 employees and had operated for 20 days . NEW : Rescuers plan to work all night at the mine , located in northern Mexico . The fate of the miners inside is unknown .
NEW : Mexico 's labor secretary says Chilean experts will visit the site Wednesday . The U.S. State Department said it was aware of reports that Chile had agreed to send a team to help with rescue efforts in northern Chile. In a Twitter post , he says the mine had 25 employees and had operated for 20 days . NEW : Rescuers plan to work all night at the mine , located in northern Mexico . The fate of the miners inside is unknown .