Tillman County EMS spokesman Ralph Washburn says the incident started around 10:30 a.m. after opening an envelope that was addressed to them.
The envelope had no return address and contained a card with a religious message inside.
Washburn says he scraped off some white-out off the card before passing it around, and when one of the other EMS workers noticed the white substance, they hit the panic button and called 911.
"You got to protect your people and you also want to protect the people that work here and the citizens of the community," Washburn says. "We wouldn't want to go out and take something that somebody had sent out into the community so we had to do the appropriate thing."
Frederick Police, a Lawton Hazmat Crew and the Lawton Fire Department responded to the scene.
Four EMS workers and two inmate workers were quarantined for a couple of hours while the scene was cleared.
Ambulance services in Frederick were shut down for a couple of hours but Washburn says an ambulance from Grandfield was called in, in case emergency services were needed.
Services to Frederick by Tillman County EMS was restored around 1:30 p.m. on Wednesday.