Bhatti, a Christian leader of the Pakistan People's Party, was gunned down in broad daylight by militants in the capital on March 3, 2011. The assassins sprayed bullets on the only minority member of the cabinet after intercepting his car.
"Threatening pamphlets by the Pakistani Taliban were thrown into the chamber of one of our witnesses recently. He is also a practising lawyer," Shamoon Gill, spokesman of APMA, told PTI.
The witness who was threatened was to appear in an anti-terrorism court on February 19.
Bhatti's brother Paul Bhatti, also a former federal minister and chairman of the All Pakistan Minorities Alliance, has left Pakistan.
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"He left on December 22 last year. Since he was the brother and pursuing the case, he was also under threat," Gill said.
By killing Bhatti hardly two months after then Punjab Governor Salmaan Taseer was gunned down by one of his police guards, militants silenced two of the strongest voices against the blasphemy law.
Bhatti had defied death threats after Taseer's assassination and said in interviews that though he was "the highest target", he would continue his work to ensure the rights of minorities.