from jana katsuyama. live at a target store in fremont after asking a security expert how a massive breach like this could have happened. reporter: that silicon valley computer expert outlined some possibilities. i did go into the store, talked to the manager, and also contacted the corporate offices in minneapolis. so far, target has issued no explanation. it appears thieves might have hit their mark. potentially stealing target customers credit card information. a worry for many shoppers tonight. i come at least four or five times. and i always buy with american express. kind of shocked that target would have this. reporter: target says it s investigating. a security expert said the breach involved nearly all of target stores nationwide. i think there s potentially three options of what happened. reporter: we asked a silicon valley security expert to explain the possible breach. he says it could have been a skimming device, physically added to a credit card read
expert how a massive breach like this could have happened. reporter: that silicon valley computer expert outlined some possibilities. i did go into the store, talked to the manager, and also contacted the corporate offices in minneapolis. so far, target has issued no explanation. it appears thieves might have hit their mark. potentially stealing target customers credit card information. a worry for many shoppers tonight. i come at least four or five times. and i always buy with american express. kind of shocked that target would have this. reporter: target says it s investigating. a security expert said the breach involved nearly all of target stores nationwide. i think there s potentially three options of what happened. reporter: we asked a silicon valley security expert to explain the possible breach. he says it could have been a skimming device, physically added to a credit card reader. more likely though, he says it could be computer hackers. they re startin