In the first incident of its kind, a woman university professor from southern Sindh province sent a parcel containing deadly anthrax powder to the Pakistani Prime Minister's Secretariat, police said on Wednesday.
The incident triggered an alarm prompting police and security agencies to launch an investigation.
No one was affected by the anthrax and investigators were yet to determine the motive of the professor from Jamshoro University, officials were quoted as saying by the Pakistani media.
It could not immediately be ascertained whether the professor had been arrested.
The parcel was received at the Prime Minister's Secretariat in October last year.
Earlier, police officials in Islamabad had said the parcel was sent to the premier's secretariat about 20 days ago.
Police launched an investigation and registered a case against "unknown persons" after a laboratory confirmed the parcel contained anthrax, they said.
A police team has been sent to Jamshoro to investigate.
The parcel was tested at a laboratory of the Council for Scientific and Industrial Research.
Though the Taliban and other militant groups have carried out scores of bombings and suicide attacks in recent years, there have been no reported instances of them using letters or parcels with poisonous materials like anthrax.
In the US, anthrax mailings rattled American public just days after the September 11, 2001 attacks.
US government scientist Bruce Ivins committed suicide in July 2008 as FBI agents were about to bring charges against him over the anthrax campaign, which killed five people and injured 17.