Those documents also reveal that a significant number of employees in DHS's intelligence office have raised concerns that the work they are doing could be illegal.

Under the domestic-intelligence program, officials are allowed to seek interviews with just about anyone in the United States. That includes people held in immigrant detention centers, local jails, and federal prison. DHS's intelligence professionals have to say they're conducting intelligence interviews, and they have to tell the people they seek to interview that their participation is voluntary. But the fact that they're allowed to go directly to incarcerated people — circumventing their lawyers — raises important civil liberties concerns, according to legal experts.

That specific element of the program, which has been in place for years, was paused last year because of internal concerns. DHS's Office of Intelligence and Analysis, which runs the program, uses it to gather information about threats to the U.S., including transnational drug trafficking and organized crime. But the fact that this low-profile office is collecting intelligence by questioning people in the U.S. is virtually unknown.

The inner workings of the program — called the "Overt Human Intelligence Collection Program" — are described in the large tranche of internal documents POLITICO reviewed from the Office of Intelligence and Analysis. Those documents and additional interviews revealed widespread internal concerns about legally questionable tactics and political pressure. The documents also show that people working there fear punishment if they speak out about mismanagement and abuses.