The D.C. jail in 2005. (James M. Thresher/The Washington Post)

More than a dozen employees at the D.C. jail complex fell ill Thursday after a package was opened containing a synthetic drug, according to D.C. fire and corrections officials.

Four workers in the mail room were immediately taken to MedStar Washington Hospital Center, and three decided to go later. Nine others were treated at the jail facility in the 1900 block of E Street SE. Officials described the symptoms as relatively minor.

Officials had said at an afternoon news conference that the substance initially tested positive for fentanyl.

But Doug Buchanan, the fire department’s spokesman, said more comprehensive tests Thursday night determined it was a synthetic cannabinoid, known as K2.

That drug, much of it produced in labs in China, has been a cheap way to get high and resulted two years ago in a spike in overdoses. The synthetic drug is addictive and causes unpredictable behavior.

The substance was in liquid form and had been addressed to an inmate at the jail, according to two District officials familiar with the case who spoke on the condition of anonymity to discuss a matter under police investigation.

Quincy L. Booth, the director of the D.C. Department of Corrections, said that all mail coming into the facility is screened and that this letter or package was encountered during that process. He said all mail deliveries and visits to inmates have been suspended until further notice. The D.C. jail houses detainees awaiting trial or convicted of low-level crimes.