KARACHI, Pakistan (CNN) -- Former Pakistani prime minister Benazir Bhutto has called the deadly terrorist attack on her convoy "an attack on democracy" and vowed it would not deter her political campaign or her fight for human rights.
Benazir Bhutto addresses reporters the day after the bombing.
Bhutto, 54, hopes to earn a third term as prime minister in January's parliamentary elections.
She returned to Pakistan on Thursday after a self-imposed, eight-year exile.
"What does the attack last night signify? The attack was more an attack on the unity and integrity of the country than on any individual or any one political party," Bhutto said at a news conference on Friday, a day after the terror attack that killed 136 people.
"It was an attack on Pakistan itself. It was an attack on their political rights, on the political process and on democracy itself. The attack last night was a message sent by the enemies of democracy to all the political parties of the country.
"It was intended to intimidate and blackmail all the political forces and elements working for democracy and human rights in the country. It was a warning not only to me and the PPP (People's Political Party) but to all political parties -- indeed, to the entire civil society."
After the blast, police retrieved the head of a possible suspect, which was being analyzed at a forensics lab.
Police said they were due to release a sketch soon and are offering about $83,000 (5 million rupees) for information leading to possible arrests.
Bhutto was traveling from the airport after returning to Pakistan when the bomber struck her convoy late Thursday. She was not hurt.
A defiant Bhutto on Friday said she did not blame the government for the attack, but complained of poor security preparations. She blamed extremists who oppose her support for Pakistan's Western allies.
No-one has claimed responsibility, and police have named no possible suspects or groups.
However, U.S. State Department officials, quoting the Pakistanis, told CNN Friday they believe there is a "strong al Qaeda connection" to the attacks. The officials said U.S. agents are helping Pakistan in the investigation.
One senior department official said the attack "bears the hallmarks" of an al Qaeda attack, and noted the group has threatened Bhutto before.
The Pakistanis have told U.S. officials that they were aware of three or four al Qaeda-related cells seeking to undertake an attack on Bhutto's return. The attack, officials said, is frightening because it shows how much freedom al Qaeda has to move around in Pakistan.
Authorities believe the suicide bomber was on foot and threw a grenade to attract attention before setting off the second, major blast, Karachi's police chief, Azhar Farooqi, told CNN on Thursday.
The bomber is believed to have acted alone. Police do not think a car bomb was involved, although nearby cars were burned.
Bhutto said streetlights on the route were not working, which hindered security guards. Authorities have promised to look into that claim.
Bhutto says she intends to continue her political campaign for prime minister, despite the potential dangers.
Her return home was made possible by Pakistan's embattled president, Gen. Pervez Musharraf, who earlier this month agreed to drop outstanding corruption charges against her and a number of other politicians as part of his own bid to stay in power.
Bhutto said Friday she is continuing to negotiate with Musharraf to promote democracy in a possible power-sharing deal. There is a parliamentary election in January in which Bhutto hopes to win a third term as prime minister.
CNN correspondent Dan Rivers said Bhutto in her speech Friday listed three other groups, in addition to the Taliban in Pakistan, that she believed posed the most danger to her and her cause: al Qaeda, the Taliban in Afghanistan and a suicide team from Karachi that she did not describe.
Meanwhile, a bomb exploded at a bus station in southwestern Pakistan Saturday, killing at least seven people and wounding 20 others, police told CNN.
The explosion occurred in Dera Bugti, an insurgency-hit tribal town in the southwestern Baluchistan province. Police have since cordoned off the area and are investigating the incident. E-mail to a friend
CNN's Syed Mohsin Naqvi and Elise Labott contributed to this report.