MEXICO CITY, April 24 (Reuters) - The majority of the people killed in Mexico's fatal flu outbreak were adults between 25 and 45 years old, a Mexican health official said on Friday.

"The majority were young adults between 25 and 45 years old," the official said under the condition of anonymity because he was not authorized to speak with the press.

Mexican officials said 20 people had been killed by the outbreak in Mexico and 1,004 people had been infected. (Reporting by Mica Rosenberg and Miguel Angel Gutierrez)