NEW: ProPublica found that over 500 U.S. citizens have been detained by immigration agents.
Forty Democratic lawmakers are now asking the Department of Homeland Security to explain why Americans have been held against their will.
Alfredo Herrera, 56, of New Carrollton was detained by U.S. Immigration and Customs Enforcement agents last Saturday, leaving his family to search for answers about why.
NEW: ProPublica found that over 500 U.S. citizens have been detained by immigration agents.
Forty Democratic lawmakers are now asking the Department of Homeland Security to explain why Americans have been held against their will.