SAN DIEGO (AP) – Nearly half of the people living in the U.S. illegally are believed to have entered the country legally and stayed on expired visas.

To combat the problem, the federal government on Thursday is launching one of its most ambitious efforts to track them.

U.S. Customs and Border Protection will begin capturing facial and eye scans of foreigners entering the country at San Diego’s Otay Mesa port of entry on foot. By February, foreigners going to Mexico on foot through the checkpoint will get scanned.

The trial run lasts through the end of June and will help determine if authorities expand biometric screening to foreigners at all land crossings into Mexico. Authorities will look at the accuracy of the cameras.

Congress has long demanded biometric screening but it poses huge financial and logistical challenges.