EL PASO, Texas — A senior Trump administration official says about 500 of the more than 2,000 children separated from their families at the border have been reunited since May.

The official said Thursday those children had been separated from their parents as a result of a "zero tolerance" policy that calls for the criminal prosecution of anyone caught crossing the U.S. border illegally. The official was not authorized to give out the number and spoke to The Associated Press on condition of anonymity.

The official says many of the children were reunited within days after being separated from their families. The official wasn't certain how many of the reunited children remained in custody with their families or how many were no longer in the country.

[Also read: Family detention facilities could reach capacity in the near future: Report]

The official says U.S. Immigration and Customs Enforcement and Health and Human Services are working to set up a centralized reunification process at the Port Isabel Detention Center near Los Fresnos, Texas.