Department of Homeland Security officials call criminal alien gangs a dangerous epidemic, but cities like Los Angeles have not allowed police officers to ask gang members, even those known to be in the United States illegally, about their immigration status.

"Illegal aliens may not be touched by LAPD officers unless that officer has a federal warrant in hand," said Heather MacDonald of the Manhattan Institute (search).

Other cities have the same policy, so-called "sanctuary laws" that bar local law enforcement departments from enforcing federal immigration law.

"We think the order, as it is, is a guarantee that the civil rights of the citizens and residents of this city are protected," said Salvador Sanabria, of El Rescate (search), a grassroots, non-profit group that provides immigration legal services in Los Angeles.

But Los Angeles and nearby Orange County are now rethinking their policies to allow cops to arrest deported felons who return illegally.

"We're going to be able to take these men and women off the streets for the federal felony of re-entering the United States illegally before they commit another crime," said Orange County Sheriff Michael Carona (search).

Opponents of the plan are warning local police that such an effort will backfire.

"If they're talking about letting their local sheriff's department also operate as INS officers, they will find they will get less cooperation when it comes to solving violent crime in Orange County," said James Lafferty of the National Lawyers Guild (search).

Lafferty said changes will be met with resistance from law-abiding illegal immigrants to help police for fear of being deported. But with up to 25 percent of California's jail population comprised of criminal illegal aliens, officers say their first duty is to protect the public.

"This is about striking a balance, protecting the safety of the people in the city and protecting the community from violent felons that are out there victimizing everyone and at the same time striking that balance so that people that are legitimate victims feel comfortable coming to us and reporting crimes," said George Gascone of the Los Angeles Police Department.

Under the new rules, L.A. officers who spot deported felons tell a supervisor, who then alerts immigration, which in turn calls a judge to issue a warrant before an arrest can be made. In Orange County, deputies are being more aggressive, tapping into a federal immigration database and making the arrests themselves.

Cities like New York, Houston and Phoenix so far have declined to modify their sanctuary laws.

Arizona state Rep. Russell Pearce has introduced a bill in the state Legislature that would allow police to start enforcing federal immigration laws.

"We have an invasion going on" that includes some of the largest, most violent gangs coming into the United States, Pearce told FOX News on Thursday, noting that an estimated 4 million illegal immigrants will enter the Untied States this year alone. "Yet law enforcement like LAPD, Phoenix PD, many, many cities in the southeast United States have these sanctuary policies ... that simply limit their ability to enforce the law."

Pearce, a former law enforcement officer himself, said since the aliens are in the United States illegally in the first place, "it's absolutely outrageous that law enforcement would run from their responsibility — this takes off the handcuffs," he said of his legislation.

"Why in the world would we think our law enforcement wouldn't have the responsibility to involve themselves in that kind of a problem?"

Click in the box near the top of the story to watch a report by FOX News' William La Jeunesse.