PHOENIX — The top prosecutors in the state’s two largest counties are expecting new challenges to Arizona’s identity theft law in the wake of a new ruling Monday by the U.S. Supreme Court.

But both Maricopa County Attorney Andrew Thomas and Barbara LaWall, his Pima County counterpart, said Monday they believe state law is sufficiently different than the federal statute that was challenged. And both said they expect to be able to continue their prosecutions of illegal immigrants.

In a unanimous decision, the nation’s high court threw out the conviction of Ignacio Flores-Figueroa an illegal immigrant from Mexico who had given his employer his own name but identification with false Social Security and alien registration cards.

Justice Stephen Breyer said out the federal statute imposes a mandatory two-year prison term on someone who “knowingly ... uses, without lawful authority, a means of identification of another person.’’

What that means, Breyer wrote for the court, is prosecutors must show that Flores-Figueroa actually knew the identity he was using — the Social Security number he had on the card — actually belonged to someone else. Absent that, the justice wrote, the case falls apart.

Thomas said he has “no doubt’’ that defense attorneys will use the U.S. Supreme Court ruling to try to undermine cases against defendants who provide cards with made-up Social Security numbers. LaWall agreed.

“They challenge everything,’’ she said.

But both said there are sufficient differences in the wording of state law to allow it to withstand a similar challenge.

The key, said LaWall, is that state lawmakers specifically authorized the prosecution of anyone who uses fake ID. The question of whether it actually belongs to someone else, she said, is legally irrelevant.

That statute makes it a crime to take, purchase, record, possess or use “any personal identifying information of another person or entity, including a real or fictitious entity’’ for any unlawful purpose.

LaWall acknowledged the term “theft’’ generally is defined as taking something from someone. But she said using a fake ID, even with a non-existent Social Security number, can be defined as theft “because the state Legislature said it is.’’

In essence, she said, lawmakers have defined the word broadly.

“It is the taking of the identity, the using of the identity,’’ LaWall said. “It is the (ITALICS) use (ROMAN) of a fictitious name for an unlawful purpose or a real name for an unlawful purpose.’’

Thomas agreed the wording of the state statute is crucial — and legally unassailable.

“The Legislature has great latitude to write the laws as it deems fit,’’ Thomas said. “Unless the Legislature passes laws that are unconstitutional, the judiciary is not able to strike them down.’’

Thomas said he believes Congress could get around Monday’s ruling by taking a page from Arizona and rewriting the federal statute to mirror the state law here. Breyer seemed to agree.

In his majority ruling, Breyer said the court was simply reading the words that Congress wrote. He acknowledged that might make prosecuting someone under the federal law difficult.

“Congress used the word ‘knowingly’ followed by a list of elements,’’ he wrote. “And we cannot find indications in statements of its purpose or in the practical problem of enforcement sufficient to overcome the ordinary meaning, in English or through ordinary interpretive practice, of the words that it wrote.’’

Thomas said the hole in the federal identity theft law underscores the importance of being able to prosecute illegal immigrants under state law.