PORTLAND — Maine State Police are investigating a security breach of a jewelry store company’s computer system that compromised an undetermined number of credit and debit cards.

Day’s Jewelers said today the suspected breach involved hackers outside the company. The Waterville-based company, which has five stores in Maine and one in New Hampshire, said it hasn’t determined how customers were affected.

The Maine State Police Computer Crimes Unit declined comment because the investigation is ongoing. Day’s has hired a computer forensics company to determine how the breach happened and identify the information that was compromised.

The Maine Credit Union League said more than 1,000 of its members who made purchases at Day’s stores in November and December have reported fraudulent activity on their cards.