The Allegheny County Jail in downtown Pittsburgh, Pennsylvania, was under lockdown Monday after nearly a dozen employees were hospitalized from illnesses caused by an "unknown odor or substance."

County police were investigating the mysterious matter, Warden Orlando Harper said in a statement. The employees, who were hospitalized Sunday night, were treated and released from a local hospital. The cause of illness was yet to be determined, officials said.

Nine corrections officers and two members of the medical staff were the victims of the incident, which bears similarities to a wave of incidents that have sickened dozens of employees at state prisons in recent months. These incidents prompted the lockdown of all 25 Pennsylvania prisons last week.

“No determination has been made as to what has caused it, but it is being treated similarly to the issue at the state correctional institutions,” Harper said, adding the jail will be on lockdown until further notice.

Officials believe the wave of incidents in recent months were caused by synthetic marijuana brought in on paper, which inmates can then eat or smoke, according to Trib Live.

Inmates were being interviewed in an effort to determine the cause of the latest incident. 

According to the warden, all cells are searched for contraband “on an ongoing basis.” The jail secured an ion scanner in January that was capable of identifying small amounts of particles like narcotics and explosives.

“As jail officials, we are constantly on alert for ways that individuals get contraband into the facility, and to try to determine what that contraband may be,” Harper said in the statement. “We consistently reinforce our policies and procedures with staff and, when appropriate, take steps to strengthen those policies and procedures.”

Speaking out on the series of illnesses in the state prison system, State Department of Corrections Secretary John Wetzel said last week the cause was believed to be synthetic cannabinoids in the form of clear, odorless liquid put on paper, rendering it undetectable.

The statewide lockdown last week included halting all non-legal inmate mail and inmate visits.