PHOENIX -- The U.S. Justice Department is filing a lawsuit challenging the constitutionality of Arizona's tough new law targeting illegal immigrants.

The planned lawsuit was confirmed to The Associated Press by a Justice Department official with knowledge of the plans. The official didn't want to be identified before a public announcement planned for later today.

The lawsuit will argue that Arizona's new measure requiring state and local police to question and possibly arrest illegal immigrants during the enforcement of other laws, like traffic stops, usurps federal authority.

Today's action has been expected for weeks. President Barack Obama has called the state law misguided. Supporters say it is a reasonable reaction to federal inaction on immigration.