Four Alabama jail inmates overdosed and were hospitalized over the weekend after ingesting a “synthetic substance” that was sent to them through the mail, a sheriff’s office said.

The Madison County Jail inmates were taken to the hospital Saturday night, Huntsville Emergency Medical Services spokesman Don Webster said. Two of them were transported in serious condition and the other two were in non-life threatening condition, WAAY-TV reported.

Madison County Sheriff’s investigators said they think the inmates “intentionally ingested” the substance, which had been sent to them through the inmate mail system, according to a agency statement.

“We are aggressively investigating this incident,” Sheriff Kevin Turner said in the statement. “It often only takes small amounts of these synthetic chemicals to cause an overdose.”

Toxicology tests were being performed to identify the substance. An update on the patients’ conditions hasn’t been released.