Yahoo Inc. says it’s investigating reports of a security breach that has purportedly exposed nearly half a million users’ email addresses and passwords.

The company said in a statement that that it was looking into “claims of a compromise of Yahoo! user IDs” but didn’t give any further detail as to the size of the reported breach or how it may have happened.

Technology news websites including CNET, Ars Technica, and Mashable cited hackers calling themselves the D33D Company as claiming responsibility for the attack, adding that data posted to the group’s website carried more than 453,000 login credentials from an unidentified Yahoo site.

A Ukraine-registered website associated with D33D Company was unreachable Thursday; an email address and a phone number attributed to the site’s registrant appeared to be invalid.