Updated: 12/20/2013 12:10 PM
Created: 12/20/2013 12:00 PM KSTP.com
By: Megan Stewart
Some people who may be victims of credit card fraud as a result of Target's security breach say they're having trouble contacting the company through its website and call centers.
Target is apologizing and says it's adding more workers to handle calls and help solve website issues.
Data connected to about 40 million credit and debit card accounts was stolen as part of a breach that began over Thanksgiving weekend, the company acknowledged Thursday.
Some furious customers are threatening to stop shopping at Target. These threats come during the busiest shopping time of the year.
Sources tell ABC News the investigation is focused on processing centers, where Target takes in the credit and debit transactions of millions of customers.
On Friday morning, the Secret Service and a private forensic team hired by the company are trying to determine how the breach took place.
They are looking at whether the theft of information was launched from the outside, possibly from overseas. They will also be investigating whether the scam was an inside job.
With thieves committing these crimes sometimes from thousands of miles away, the Secret Service says there are just a few critical things consumers can do. These include regularly checking credit reports and billing statements.
In 2006, hackers stole personal information from nearly 46 million customers of popular stores including TJ Maxx and Marshalls.
The Associated Press contributed to this report.