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Hundreds of used syringes and blood vials in biohazard containers ended up in the Arkansas River after a private company was hired to dispose of them, Oklahoma investigators said Monday.

The five containers were spotted Saturday in the Pawnee County section of the river near Ralston, west of Tulsa, by a group of friends who were out on a boat.

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"There's no telling how long this has happened or if it's happened more than once," one of them, Taylor Smyth, told NBC station KFOR of Oklahoma City.

"I don't feel comfortable swimming now," Smyth said.

The Pawnee County Sheriff's Office said biohazard investigators were working to trace the materials — some of which bear labels with patients' names on them — back to their source. Sheriff Mike Waters warned people Monday to look carefully for hazards before entering the water.

Five of the biohazard containers, filled with blood vials and used syringes, were found Saturday in the Arkansas River in Pawnee County, Oklahoma. KFOR-TV

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Vials of blood and used syringes found in Oklahoma in the Arkansas River. Disturbing much? http://t.co/0IhDDeCLFN — Stephen Bowers (@KOAAsbowers) March 16, 2015

— M. Alex Johnson