CHICAGO (AP) — An official says operational changes at the U.S. Postal Service resulted in the delayed delivery last month of 5,000 prescriptions filled by Cook County Health’s mail-order pharmacy.

Chief Medical Officer Dr. Claudia Fegan says the pharmacy normally fills more than 20,000 prescriptions monthly. Fegan said Monday 23% of those prescriptions were delayed, up from less than 1% in March.

She said there were also delays in April and May, reaching 18% in June. Postmaster General Louis DeJoy appeared Monday before a U.S. House committee hearing on the operational changes.

Democrats said the changes under DeJoy’s watch are causing widespread delays, but Republicans dismissed the worries as unfounded and part of a Democratic “conspiracy” against Trump.