Although immigration is one of America's most divisive, visceral, and hotly debated issues, the public rarely gets a close look at the vast law enforcement network that every year detains more than 400,000 suspected illegal immigrants. Courts often operate inside prisons, far from view. Immigration officials play by rules that would not be permitted for the police or the FBI. Here is a system heavily shielded from public scrutiny. Reporting even routine activities is a challenge. Boston Globe reporters Maria Sacchetti and Milton J. Valencia, however, penetrated the wall of secrecy. Their three-part series, “Justice in the Shadows,” revealed a dysfunctional and largely unaccountable system that locks up people who pose little threat while releasing dangerous criminals back to US streets because their home countries won't take them back. The results, Sacchetti and Valencia showed, at times can be deadly for Americans and foreigners alike. The reporting was anything but quick or easy. Sacchetti and Valencia filed more than 20 Freedom of Information Act requests to federal agencies that comprise the immigration system. Nearly all of them were partially or wholly denied, purportedly to protect the privacy of the immigrants. With the federal government blocking the way, Sacchetti and Valencia found other avenues to document what was happening inside this Byzantine system, investing a year to do so. The effort to shed light on the immigration system continues: The Globe has filed a federal lawsuit against the Department of Homeland Security to force the agency to reveal the names of more than 8,000 criminal foreigners released in the US because they couldn't be deported.