Benazir Bhutto’s widowed husband accused members of Pakistan’s ruling regime of involvement in his wife’s killing and called Saturday for a U.N. investigation, as British officers aiding Pakistan’s own probe pored over the crime scene.

“An investigation conducted by the government of Pakistan will have no credibility, in my country or anywhere else,” Asif Ali Zardari, the effective leader of Bhutto’s opposition party, said in a commentary published in The Washington Post. “One does not put the fox in charge of the hen house.”

Calls for an independent, international investigation have intensified since the former prime minister was killed Dec. 27 in a shooting and bombing attack after a campaign rally. Opposition activists denounced the government’s initial assessment that an Islamic militant was behind the attack and that Bhutto died, not from gunshot wounds, but from the force of the blast.