PORT-AU-PRINCE, Haiti — A cholera outbreak that has killed more than 300 people in Haiti matches strains commonly found in South Asia, the U.S. Centers for Disease Control and Prevention said Monday.

The finding is a significant step toward answering one of the most important questions about the burgeoning epidemic: How did cholera, a disease never confirmed to have existed in Haiti, suddenly erupt in the vulnerable country’s rural center?

It also intensifies the scrutiny of a U.N. base that is home to recently arrived Nepalese peacekeepers, built on a tributary to the Artibonite River. Cholera has been detected in the waterway, and most of the cases have been among people who live downriver and drank from the Artibonite.

Speculation among Haitians has increasingly focused on the base and the troops from Nepal, where cholera is endemic and which saw outbreaks this summer before the arrival of the current contingent of troops.

In an unannounced visit to the base last week and a tour of the facility given by peacekeepers Sunday, The Associated Press found questionable sanitation conditions.

The U.N. has defended its sanitation practices and denied that it was a source of the infection. A spokesman said the agency was looking into the matter Monday following the announcement.