Throughout years of legal battles over the country's immigration system, a basic question has received relatively little attention: Is information the government stores on the country's millions of immigrants reliable?

That will change this week with the opening of a federal trial in Los Angeles.

At issue is the hodgepodge of databases that U.S. Immigration and Customs Enforcement officers rely on when issuing so-called detainers, which are requests made to police agencies to keep people who have been arrested in custody for up two days beyond the time they would otherwise be held. Tens of thousands of the requests are made each year to allow ICE agents additional time to take people suspected of being in the country illegally into federal custody for possible deportation.

The class-action lawsuit, which represents broad categories of people who have been or will be subjected to detainers, alleges the databases that agents consult are so badly flawed by incomplete and inaccurate information that ICE officers should not be allowed to rely on them as the sole basis for keeping someone in custody.

Although police in California do not honor these ICE requests because of earlier court rulings that found them unconstitutional, agencies in other parts of the country continue to enforce them.

Los Angeles Times