"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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The men are said to be linked to a car from which an attacker threw a grenade on Thursday night, seconds before the twin blasts.
"Police have taken three people from the south of Punjab province for questioning. They were taken to Karachi and it can be that they hold clues," an official said.
"They were picked up in connection with a suspect vehicle from which one of the attackers is believed to have thrown the grenade."
Investigators also questioned religious activists in Karachi jails for possible information on the blasts.
Bhutto has blamed al-Qaeda and Taliban fighters for the bombing, but also suggested that government or military officials could have been involved.
She questioned why street lighting had been off during part of her procession.
Bhutto said on Friday that she had sent Pervez Musharraf, Pakistan's president, the names of three people she accused of involvement in the Karachi blasts.
"I have shared the names with General Musharraf and one of the people is someone that they are [already] watching," she told the BBC in an interview.
She refused to give their names.
Sources in her Pakistan People's party (PPP) said they had included senior army officials, but would not give further details.
But Tariq Azim, Pakistan's deputy information minister, has dismissed accusations that officials may have been complicit in the attack on Bhutto's convoy.
He said that the "best possible security" had been provided before her return to the country.
Azim said: "The trauma of the attack has made them say things which probably in coolness of things they will not repeat.
"People's names have been mentioned and names have been hinted at without giving any reason or without giving any proof of their involvement, and that is unfair."
The government's denial came as a bomb reportedly ripped through a bus in the main market of Dera Bugti in Baluchistan.
The province has seen sporadic violence by ethnic Baluch tribesmen demanding greater political autonomy and a larger share of profits from the province's plentiful natural resources.
"There was a bomb explosion in the main bazaar and seven people were killed and six are wounded," Hazoor Baksh, a local police officer, told the AFP news agency.
No one immediately claimed responsibility for Saturday's attack.
There have been fears after Thursday's assassination attempt on Bhutto that further attacks could damage Pakistan's already fractured political landscape.