An investigation is underway to find out how the inmates at Westchester County Jail got a hold of a synthetic form of marijuana.

A lockdown was placed on the facility after more than a dozen inmates apparently overdosed on drugs between Sunday and Monday. Authorities suspect the cause was a form of synthetic marijuana known as K2 or Spice.

By Monday night, a total of 13 prisoners showed symptoms of an overdose, including nausea and vomiting. While the lockdown has since been lifted, there's still the question of how the drugs got inside.

"Contact visits with people from the outside, something can be brought in, and that's something I've discussed with the commissioner to potentially limit contraband from coming in," says Westchester County Executive Rob Astorino.

Two inmates, Deron Strange, 31, of Port Chester, and James Graziano, 30, of Mount Vernon, were charged Monday with promoting prison contraband.

Four other inmates have also been arrested. They have been identified as Kevin Murph, 33; Paul Overton, 33; Franklin Sanchez, 27; and Drew Senior, 21.

Officials say stricter rules may be implemented when it comes to outside visitors, but there is no word yet on what measures will be taken.