Hilton Worldwide Holdings plans to investigate allegations that hackers accessed data records at gift shops and restaurants in a large number of Hilton hotels across the United States.

Blogger Brian Kerbs, an internet security expert, said in a blog post on Friday that Visa sent confidential alerts to financial institutions regarding breaches at a business establishment between April 21 and July 27.

Kerbes said sources at five different banks confirmed that the credit cards included in the alert were all used at Hilton properties, The report stated that several unnamed sources in the financial services sector informed Kerbs that the breach is ongoing and may date back to November 2014.

A Hilton spokesperson said in a statement “It is unfortunate that the potential use of counterfeit credit cards is all too common for any company in the market today. We take any potential issue seriously and are looking into this matter.” The spokesperson declined to provide further details.