The suicide blasts struck Bhutto's homecoming parade in Karachi on October 18 killed 139 people and wounded scores more, but Bhutto escaped unharmed.
"Perhaps Bhutto should have listened to Musharraf and delayed her return to Pakistan until the volatile security situation was better"
ndur5, Irving, US
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Hundreds of thousands of supporters had turned up to receive her after she arrived in Pakistan, ending eight years of exile.
Mohtarem said Mirza would be officially assumed the post on Thursday.
Meanwhile, police have questioned 16 people over the blasts. The men were not considered suspects, Mohtarem said.
It was not immediately clear why they were being questioned.
Mohtarem said no one had yet been arrested over the attacks but he insisted the investigation was progressing well.
"We have got certain leads and we are working on them," he said.
Bhutto has been under heavy security in Karachi since the attacks.
However, she has vowed to stay in Pakistan to campaign for the general elections set for January.
Bhutto went into self-imposed exile in 1999.
Corruption charges that forced her to flee Pakistan were dropped earlier this month, paving the way for her return.