MAHNOMEN, Minn. — Authorities in northwestern Minnesota are trying to determine the source of a substance on cash at a convenience store that sent seven employees to the hospital.

The Mahnomen County Sheriff's Office says an employee of Pump 24 felt a burning sensation after taking cash from a customer on Monday.

Sheriff Doug Krier says a paramedic who was in the store told the employee to thoroughly wash her hands, but she was still feeling the effects and drove herself to the hospital. Other employees were sent to the hospital as a precaution.

Krier says the store was closed and sealed off while a hazardous materials team checks the building.

KFGO Radio reports the customer who gave the clerk the money has not been found.