A major American credit reporting agency entrusted to safeguard personal financial information said Thursday hackers looted its system in a colossal breach that could affect nearly half the US population as well as people in Britain and Canada.

Equifax said that a hack it learned about on July 29 had the potential to affect 143 million US customers, and involved some data for British and Canadian residents.

The Atlanta-based company disclosed the breach in a release that did not explain why it waited more than a month to warn those affected about a risk of identity theft.