What some advocates say undocumented people in US should know before registering with government

SANTA CLARA COUNTY, Calif. -- All non-citizens who are in the United States will be required to register with the government.

On Thursday, a federal judge allowed the Trump administration to move forward with enforcement of an already existing law - anyone in the country illegally must register.

"I think that many people are going to register," said Bill Hing, professor of Law and Migration Studies at the University of San Francisco.

Registration will be mandatory for anyone 14-years-old and older. They will have to provide fingerprints and addresses.

"A lot of the questions have to do with - did you commit a crime? Or have you committed an act that is criminal that you've never been prosecuted for?," he said.

The registration policy dates back more than 80 years... the federal government arguing it just wants to enforce a law long on the books.

RELATED: Everyone in the US illegally must register with the federal government beginning Friday

But opponents say the registration requirement hasn't been universal since it was first introduced.

If someone who is undocumented does not register, it's considered a federal crime, a misdemeanor.

"Will anybody ever get prosecuted for a misdemeanor is the real question, and I don't think so. I think somebody that is caught who is undocumented, they're going to be deported because they're undocumented," Professor Hing said.

"I don't think the law is actually ever going to be enforced, I think that it's a mechanism to get undocumented people to expose themselves."

In Santa Clara County, approximately 140,000 people who are undocumented call the area home.

The county says over 160,000 people are in are mixed status households, which means some family members are citizens and others are undocumented.

"So we are seeing another attempt to terrify, and terrorize the immigrant communities that have made our nation what it is," said Huy Tran, the Executive Director of SIREN. It's a community-based organization based in the Bay Area and Central Valley.

"For now, I think the best thing for anyone and everyone in our community to do, is not to register until they've had a chance to talk to their attorney, or to get in touch with a legal services organization focused on immigration," he said.

The registration process also applies to Canadians who have been in the U.S. for more than 30 days.