Pakistani police were questioning three people on Sunday over an attack on former premier Benazir Bhutto's homecoming procession that killed 139 people, an investigator said.
The men were linked to a car from which an attacker threw a grenade in Karachi on Thursday night, seconds before a suicide bomber blew himself up in a crowd of hundreds of thousands of people, he said.
Investigators also quizzed seven militants in jails in Karachi for possible information on the blasts, added the police official, who has investigated several other attacks in the city.
"Police have taken three people from the south of Punjab province for questioning. They were taken to Karachi for questioning and it can be that they hold clues," the official told AFP on condition of anonymity.
"They were picked up in connection with a suspect vehicle from which one of the attackers is believed to have thrown the grenade," he said.
Several hundred protesters burned tyres in the streets on Saturday and pelted passing vehicles with stones for a second day in pro-Bhutto neighbourhoods, AFP photographers said.
Pakistani authorities were earlier probing a list of three possible suspects that Bhutto sent to President Pervez Musharraf.
The 54-year-old has blamed militants for the attack and said she did not believe that the "state or government" was involved, but sources in her party said the list included senior army officials, without elaborating.
Bhutto has said she received a warning prior to her return from Dubai about members of Al-Qaeda, Pakistani and Afghan Taliban and a Karachi-based militant group who may plan to attack her.
She has also accused Islamist supporters of late military ruler Zia-ul-Haq of being behind the blasts, the worst suicide bombing in Pakistan's history.
He overthrew Bhutto's father, Prime Minister Zulfiqar Ali Bhutto, in 1977 and had him hanged two years later.
The United States, which has backed a proposed power-sharing deal between key "war on terror" ally Musharraf and the Oxford- and Harvard-educated Bhutto, has offered its help in the investigation.
Police released a photograph of the bomber's head and said they were pushing forward with the probe but had made no arrests. Musharraf pledged to bring the culprits to justice in a phone call to Bhutto on Friday.
Bhutto has pledged to stay in Pakistan to combat militancy and fight general elections in January, seen as a key step to returning the Islamic republic of some 160 million people to civilian rule.
But the attack on her motorcade has cast doubt over her previous plans to tour the country to whip up support ahead of the polls.
Her party said she would soon visit the tomb of her father in her family's ancestral village of Larkana, deep in southern Sindh province, after consultations with senior party leaders at her home in Karachi.
The explosions came hours after Bhutto had flown home, sobbing as she set foot on Pakistani soil for the first time since 1999 after shrugging off warnings of militant attacks.
Bhutto had returned from eight years in self-imposed exile after Musharraf dropped corruption charges against her in the hope her popularity could shore up his grip on power.
She had mostly worked out a power-sharing deal with him, but his re-election as president earlier this month is now being challenged in the courts, as is the graft amnesty.