The doctor who treated Benazir Bhutto talks about her condition when she arrived at the hospital.
ISLAMABAD, Pakistan (CNN) -- Benazir Bhutto died as a result of a fractured skull after hitting her head on part of her car's sun roof, not as a result of a bullet or bomb shrapnel, a spokesman for Pakistan's Interior Ministry said Friday.
Asif Ali Zardari, in white cap on left, helps carry the coffin of his wife Benazir Bhutto during Friday's burial.
1 of 6 more photos » Nothing entered the opposition leader's head, said spokesman Brigadier Javed Iqbal Cheema.
At a news conference, Cheema showed images of Bhutto in a car, standing up through an open sunroof, looking out at the crowd as she was about to be driven away.
When the gunshots rang out and the explosion occurred, Bhutto "fell down or perhaps ducked" and apparently hit her head on a lever, Cheema said, adding that the lever was stained with blood. Watch how investigators think Bhutto died »
Earlier, the ministry said in a report carried by the state-run news agency Associated Press of Pakistan that she was killed by shrapnel from the suicide bomb that was detonated as she stood up through the sun roof while being driven away from a political rally.
The blast killed at least 28 more people and at least 100 were wounded.
And it was initially reported on Thursday that the two-time former prime minister had died due to prior gun shots fired by the bomber.
The Interior Ministry also revealed Friday that it had proof showing that al Qaeda was behind Bhutto's assassination.
Brigadier Javed Iqbal Cheema said the government had an intelligence intercept in which an al Qaeda militant "congratulated his people for carrying out this cowardly act."
However, that claim has not appeared on radical Islamist Web sites that regularly post such messages from al Qaeda and other militant groups.
The Interior Ministry also earlier told Pakistan's GEO-TV that the suicide bomber belonged to Lashkar-e-Jhangvi -- an al Qaeda-linked Sunni Muslim militant group that the government has blamed for hundreds of killings.
U.S. officials believe that a Taliban leader from Afghanistan, Baitullah Mahsud, may be the person behind the assassination.
Bhutto was laid to rest in a chaotic funeral at her ancestral home Friday after violent scenes erupted across Pakistan following her death a day earlier.
Bhutto was interred alongside her father in the southern Pakistan town of Garhi-Khuda Baksh.
Hundreds of thousands of people in the surrounding streets almost brought the procession to a standstill before it finally reached the Bhutto family's mausoleum.
The throngs of her grieving supporters crushed up against the flag-draped coffin, while minor scuffles also broke out.
Violence had earlier erupted in Pakistan in the hours before Bhutto's funeral started, with at least nine people reported killed and banks, train stations and cars torched. Watch video from Bhutto's funeral procession »
Bhutto's body arrived in the hours before dawn at Garhi-Khuda Baksh after a long journey by plane, helicopter and ambulance. See images of Pakistan mourning Bhutto. »
The opposition leader's family -- her husband Asif Ali Zardari and three children -- accompanied the body aboard a Pakistani Air Force C-130 transport plane to Sukkor but traveled by bus from there to Larkana and on to Garhi-Khuda Baksh.
Another former prime minister, Nawaz Sharif, told CNN on Friday that he had planned to attend Bhutto's funeral, but was advised not to by Zardari, who cited security concerns.
"He said that we must not come today in view of these inadequate security arrangements," Sharif said. "The security arrangements are far from satisfactory."
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In the aftermath of the assassination, the prime minister's office has launched a judicial inquiry and the Ministry of the Interior is setting up a police inquiry, according to Information Minister Nisar Memon.
Memon said no decision had been made to postpone parliamentary elections scheduled for January 8.
"We remain on course," he said.
Sharif, who visited the hospital to pay his respects to Bhutto, later announced that he and his party would boycott the elections.
Bhutto, who was campaigning for the elections, had completed an election rally minutes earlier and was leaving the rally site, Rawalpindi's Liaquat Bagh Park, at the time of the attack. What impact could Bhutto have had in Pakistan? »
Her father and former prime minister, Zulfiqar Ali Bhutto, was hanged in the same northern city in 1979.
As a shocked Pakistan absorbed the news of Bhutto's death, authorities called for calm and asked residents to stay inside.
Many obliged, shuttering shops or rushing home from work and surrendering the streets to protesters who set fire to banks, shops and gas stations, blocked roads and pelted police with rocks, Pakistani media reported.
At least five people were killed in Karachi in the violence, GEO-TV reported, and dozens more were wounded. Police in Khairpur fired on an angry mob, killing two people, the station reported, and two more people were killed in Larkana.
"It's all mayhem everywhere," Shehryar Ahmad, an investment banker in Karachi, told CNN by telephone. "There's absolutely no order of any kind. No army on the streets. No curfew."
In Sindh province, where Karachi is located, police said demonstrators had burned a dozen banks, set two train stations on fire, along with three trains. Since Thursday, 240 vehicles have been burned.
Because of the violence, paramilitary forces in Sindh were told to "shoot on sight" anyone causing civil disturbances, a spokesman for the Pakistan Rangers said.
Local media reported that in some areas, police were on the streets but were avoiding direct confrontation with the mobs, not wanting to inflame an already tense situation.
But by Friday morning, Pakistani media reported that an uneasy calm had spread across the shaken country, now marking a three-day period of mourning declared by President Pervez Musharraf.
Bhutto led Pakistan from 1988-1990 and 1993-96, but both times the sitting president dismissed her amid corruption allegations. She was the first female prime minister of any Islamic nation.
A terror attack targeting her motorcade in Karachi in October killed 136 people on the day she returned to Pakistan after eight years of self-imposed exile.
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