ISLAMABAD, Pakistan (CNN) -- Benazir Bhutto died from a fractured skull caused by hitting her head on part of her car's sunroof as a bomb ripped through a crowd of her supporters, a spokesman for Pakistan's Interior Ministry said Friday.

Members of Bhutto's Pakistan People's Party in the northwestern city of Peshawar protest her assassination. more photos »

"When she was thrown by the force of the shockwave of the explosion, unfortunately one of the levers of the sunroof hit her," said spokesman Brigadier Javed Iqbal Cheema.

The explanation is the latest from the Interior Ministry. It initially said Bhutto was killed by shots fired by the bomber, and then, via the state-run Associated Press of Pakistan, it said the cause of death was a shrapnel injury.

But Farzana Raja of Bhutto's Pakistan People's Party told CNN the government's explanation is "a pack of lies," she told CNN. Raja also accused the government of a "total security lapse."

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 the shots, then a blast »

The blast killed at least 28 more people and at least 100 were wounded.

The Interior Ministry also revealed Friday that it had proof showing that al Qaeda was behind Bhutto's assassination.

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 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, Baitullah Mahsud, may be the person behind the assassination.

Bhutto was laid to rest in a chaotic funeral at her ancestral home of Garhi-Khuda Baksh on Friday after violent scenes erupted across Pakistan following her death a day earlier. Watch video from Bhutto's funeral procession »

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. See images of Pakistan mourning Bhutto. »

Violence 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.

Bhutto's body arrived in the hours before dawn at Garhi-Khuda Baksh after a long journey by plane, helicopter and ambulance.

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.

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.

Bhutto, who was campaigning for the elections, had completed an election rally 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.

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.

But by Friday morning, Pakistani media reported 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.

Bhutto had been critical of what she believed was a lack of effort by President Musharraf's government to protect her. View timeline » E-mail to a friend

All About Pakistan • Pervez Musharraf • Nawaz Sharif • Benazir Bhutto