Governor Jan Brewer of Arizona has signed into law an immigration bill seen as one of the toughest in the United States.
The bill will require state police to question people about their immigration status if there is "reasonable suspicion". The bill also makes it a crime under state law to be in the US illegally.
Governor Brewer signed the bill into law live on television, saying it "protects every Arizona citizen".
She said the measure would strengthen border controls in the state, which borders Mexico.
The governor also said she had to act because the federal government had failed to tackle illegal immigration.
President Barack Obama said he had instructed the Justice Department to examine if the bill was legal.
The BBC reports civil rights groups have said they will mount a legal challenge to have the law overthrown on the grounds that it paves the way for widespread discrimination against Hispanics.
The bill takes effect in 90 days.