By Tal Kopan, CNN

(CNN) -- Even as the Trump administration is asking Congress to approve a tough overhaul of the nation's immigration laws, the Department of Homeland Security is also quietly exploring ways it could transform the US immigration system on its own.

The department has been examining a range of subtle modifications to immigration policies that could have major consequences, including limiting protections for unaccompanied minors who come to the US illegally, expanding the use of speedy deportation proceedings, and tightening visa programs in ways that could limit legal immigration to the US, according to multiple sources familiar with the plans.

None of the policies being explored are finalized, according to the sources, and are in various stages of development. Any of them could change or fall by the wayside. Some of them are also included at least in part in the wish list of immigration priorities that President Donald Trump sent to Congress this week, and it's unclear whether the administration will wait to see the results of negotiations over the future of the Deferred Action for Childhood Arrivals (DACA) program that President Donald Trump has chosen to end.

Still, the proposals under consideration illustrate the extent to which the administration could attempt to dramatically change immigration in the US through unilateral executive action.

"Do you think Obama did a lot? That's my answer," said one former DHS official when asked how transformative the change could be. "They could do quite a bit."

DACA itself was an example of how former President Barack Obama, frustrated with congressional inaction, sought to use executive authority to take action on immigration, putting in place the program to protect young undocumented immigrations brought to the US as children from deportation in 2012.

But the administration is now exploring rolling back more Obama-era policies, and changing even older systems.

DHS did not respond to a request for comment about the policies being explored or its process.

Targeting protections for unaccompanied minors

One effort underway is exploring what can be done about unaccompanied children (UACs), a category of undocumented immigrants who are caught illegally crossing the border into the US, are under age 18, and are not accompanied or met by a parent or guardian in the US. Those UACs, by law and legal settlement, are handed over to the Department of Health and Human Services for settling in the US, given protections from expedited removal proceedings and given special opportunities to pursue asylum cases in the US.

DHS and the Department of Justice have been exploring options to tighten the protections for UACs, including no longer considering them UACs if they're reunited with parents or guardians in the US by HHS or once they turn 18.

In a previously unreported memo, obtained by CNN, the general counsel of the Executive Office of Immigration Review, which manages the nation's immigration courts, wrote in a legal opinion that the administration would be able to decide a UAC was no longer eligible for protections -- a sea change in the way the 2008 law granting those protections has been interpreted.

The Trump administration has portrayed the UAC protections as a loophole in the law that can be exploited by gangs, though experts have testified before Congress that the minors under the program are more likely to be victimized by gangs in the US due to a lack of a support network than to be gang members. The administration also has sought to crack down on parents who pay smugglers to bring their children into the US illegally, even to escape dangerous situations in Central America.

The White House also asked Congress to amend the 2008 law to restrict UAC protections.

In previously unreported comments made last month at a security conference in Washington, acting Immigration and Customs Enforcement Director Tom Homan said that ICE is actively looking at the adults HHS places UACs with, and if they are in the US illegally, they will be processed for deportation -- and if a smuggler was paid, they could be prosecuted for human trafficking.

"You cannot hide in the shadows, you can't be an illegal alien in the United States, have your undocumented child smuggled at the hands of a criminal organization, and stay in the shadows," Homan said. "We're going to put the parents in proceedings, immigration proceedings, at a minimum. ... Is that cruel? I don't think so. Because if that child is really escaping fear and persecution, he's going to stand in front of an immigration judge to plead his case, his parents should be standing shoulder to shoulder with him. I call that parenting."

DHS is also continuing to weigh its options to expand the use of expedited removal more generally -- a speedier process of deportation that bypasses a lengthy court process in particular cases -- as authorized by Trump's January executive order on immigration.

Legal immigration tightening

Other efforts in the works include ways to tighten legal avenues to come to the US.

Two policies being looked at are the subject of litigation in the DC Circuit court -- work authorizations for spouses of high-skilled visa holders and an expansion of a program that allows STEM students to stay in the US an extra two years for training.

Both policies were challenged in the courts, and now the administration is considering whether to roll them back.

On the spousal authorizations, DHS told the court as much in a filing last month, asking for extra time for the DHS review to finish.

That filing points to a DHS review of "all" of the agency's immigration policies, citing the President's Executive Order to "buy American and hire American."

"Executive Order 13,788 is an intervening event necessitating careful, considered review of all of DHS's immigration policies to ensure that the interests of US workers are being protected," the attorneys wrote, citing the order's instructions to create new rules, if necessary, "to protect the interests of United States workers in the administration of our immigration system."

DHS has also moved to tighten asylum claim credibility thresholds, and is exploring asking Congress for more authority to do so. Another target is reportedly cultural exchange visas, which according to The Wall Street Journal are also under scrutiny after the "hire American" order.

Further unilateral moves wouldn't even require policy changes, immigration attorneys fear. Attorneys who represent immigration clients fear that simply by slowing down the visa process, DHS could substantially decrease the number of immigrants admitted to the United States. US Citizenship and Immigration Services announced this summer it would begin requiring interviews for all green card applicants on employment and refugee grounds, and that it would roll out required interviews for other categories over time, adding a substantial and potentially lengthy hurdle to achieving legal permanent residency.

"If the wait time for naturalizations increases by three months, USCIS can naturalize 25% fewer people per year, which would mean millions of people over a four-year period," said Leon Fresco, an immigration attorney and former Obama administration DOJ official. "Even without a policy change, the administration (can accomplish) dramatic reductions to legal immigration through increases in processing times and taking a hawkish approach to finding reasons for denials of immigration applications."

DHS pointed CNN to statistics showing no increase in the rate of denials of immigration applications, though the backlog of pending applications has grown steadily over the past two years.

Internal jockeying

Sources familiar with the inner workings of DHS describe an environment where political appointees and policy staff with strongly held opinions circulate ideas that sometimes reach the press before front office and secretarial staff are even aware of the discussions.

While political appointees and career officials are not described as butting heads, some of policy ideas do end up moderated by career employees on practical grounds. One source also described some employees of USCIS, which administers DACA, as getting emotional when the plan was made to end the program.

"Once it gets to a senior level, there are pretty robust discussions," another source familiar said. "And once it gets to that level there are folks with ideas, and then folks who have been around for a while who say, 'That won't work.'"

Those competing ideas are then ultimately decided on by the secretary and high-level decision makers, though sources say political appointees are sometimes in a position to have influence over what information flows to the front office and top officials.

"The secretary and the decision makers end up with that (dynamic)," the source said.