Several large American companies, including electronics giant Best Buy, have warned customers their personal information may have been compromised over the April 1 weekend.

The companies in question cautioned their customers to remain alert for fraudulent emails, after computer hackers gained access to a marketing database, possibly stealing millions of customer names and email addresses.

The breach of security took place at Epsilon, a Dallas-based marketing firm that manages email communications for more than a dozen large companies, ranging from online banks to retail stores.

Epsilon admitted Friday their database had been breached, which allowed hackers to access information limited to names and emails. The company assured consumers more potentially damaging information such as credit card numbers or bank accounts were not stolen.

The largest risks from the security breach include hackers using the email lists to target consumers with spam, or potentially the usage of phishing scams – false emails intended to steal log-in information from online banking customers.

Phishing scams generally send emails to consumers asking them to enter their banking credentials into a fraudulent site, which then records the information and uses it to log into real banking sites to steal money.

Among the affected companies are banks like Capital One, Barclays, and Citigroup. Retailers like Best Buy, TiVo and Walgreen were also hit in the breach.

Also at risk are thousands of students, after The College Board, which organizes and runs the SATs, warned a hacker may have gained access to student emails.

Epsilon sends over 40-billion emails annually, and has more than 2,500 clients.