MEXICO CITY — The U.S. government is expanding a program to capture the biometric data of tens of thousands of Central Americans and other migrants arrested in Mexico, gaining unprecedented access to Mexican immigration jails to identify criminals, gang members and potential terrorists long before they reach the U.S. border.

Operating in detention facilities in southern Mexico and here in the capital, Department of Homeland Security officials have installed scores of screening terminals to collect migrants’ fingerprints, ocular scans and other identifying features, including tattoos and scars.