Arizona authorities battling the Obama administration over the state's new immigration law may face an unforeseen obstacle in enforcing the measure: While local police can arrest illegal immigrants, only the federal government can deport them.

Because the federal immigration-enforcement agency has suggested it won't necessarily cooperate in helping meet the law's goal, local police could be forced to choose between holding detainees in their own jails or releasing them.

Inmates seen Tuesday in tents set up to house as many as 110 illegal immigrants, who could be arrested under Arizona's new state law, at the Maricopa County Sheriff's Office jail in Phoenix. Associated Press

The Arizona law, which takes effect Thursday, makes it a state crime to be in the U.S. illegally and requires police who stop people for such routine infractions as speeding to check their immigration status if there is "reasonable suspicion" they are here unlawfully. The police can detain them while clarifying their status, but deportation can only be conducted by federal authorities.

On Tuesday, Matthew Chandler, a spokesman for the Department of Homeland Securtity—which houses U.S. Immigration and Customs Enforcement—said the priorities for federal enforcement apply to recent border crossers, felony re-entry cases and serious criminals who pose a public-safety risk.

Police officers in Arizona and across the U.S. currently have the capability to contact ICE to verify the immigration status of people they stop. The agency then confirms whether a person is deportable. If the person doesn't fall into a priority category, they might be issued a notice to appear before an immigration judge but wouldn't be detained by the federal government. That means the individual could remain in the country for months or years while they fight their removal.

Arizona is home to about 460,000 illegal immigrants, according to the DHS. If the new law goes into effect—a challenge by the Obama administration is pending before a judge—police could shower ICE with calls. Without its cooperation, police likely would have to release suspects, because the law doesn't give them the authority to detain an individual indefinitely based on suspicion of unlawful presence in the U.S.

To achieve the law's stated goal of attrition through enforcement, "ICE needs to play with us," said John Harris, president of the Arizona Association of Police Chiefs. "Without ICE, it makes it much more difficult for us to arrest people."

Asked about its plans regarding Arizona's new law, the agency said that "ICE is focused on smart, effective immigration enforcement that focuses first on criminal aliens who pose a threat to our communities." It added that the outcome of the Justice Department's challenge to the law—on grounds that immigration enforcement is the purview of the federal government—"will inform the government's actions going forward."

ICE now works with local law enforcement in Arizona and other states. For example, illegal immigrants who have committed serious crimes normally are turned over to the agency after completing sentences for state crimes. The agency also has agreements with more than 70 localities that empower about 1,200 personnel, who have received special training, to verify the status of immigrants.

Arizona's law would require the state's 15,000 local law-enforcement officers to check the status of those they suspect are in the U.S. illegally.

Juan Pedrosa, an immigration -policy researcher at the nonpartisan Urban Instititute, said ICE's response would be crucial because it is being closely watched by other state lawmakers seeking to promote the arrest and deportation of large numbers of illegal immigrants.

About 11 million illegal immigrants are believed to live in the U.S. In a June 30 memo to ICE employees, agency chief John Morton said it had the capability to remove "less than 4% of the estimated illegal alien population in the United States."

A record 387,790 people were deported from the U.S. in the fiscal year ended Sept. 30. ICE removed 81,429 in Arizona, including 23,550 criminals. Mr. Morton has said he opposes Arizona's law because it isn't the role of states to draft their own responses to illegal immigration.

In a declaration with the federal lawsuit, David Palmatier, head of ICE's Law Enforcement Support Center, said an increase in queries from Arizona "will delay response times…and risks exceeding the capacity of the center to respond to higher priority requests for criminal alien status determination."

—Laura Meckler contributed to this article.

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