There are differing accounts of what caused Ms Bhutto's death
A team of Scotland Yard investigators has arrived in Pakistan to assist in the inquiry into the assassination of former Prime Minister Benazir Bhutto.
Earlier, President Pervez Musharraf said he was "not fully satisfied" with the investigation into the killing.
But Mr Musharraf said he did not believe government or intelligence agencies had tried to "hide secrets" after Ms Bhutto's murder last Thursday.
Ms Bhutto's party has called for a wider inquiry by the United Nations.
"The team from Scotland Yard has arrived and they left the airport in a private vehicle," news agency AFP quoted a senior official at Islamabad airport as saying.
According to reports, the detectives were mobbed by journalists at the airport, but they left without speaking.
On Thursday, President Musharraf told a news conference that uncertainty remained over the exact cause of Ms Bhutto's death.
An initial government report said she was killed when a bomb blast caused her head to hit the bullet-proof vehicle she was travelling in at the time.
That account has been rejected by Ms Bhutto's supporters.
President Musharraf says he is not satisfied with the probe
"One should not give a statement that's 100% final. That's the flaw that we suffer from," Mr Musharraf said, adding that fresh evidence had come to light.
He said there had been no lapse in security and that Ms Bhutto had committed a grave breach of security by standing up through the car's sun roof.
Referring to the team of detectives being sent by the UK, he said: "We needed more experience, maybe more forensic and technical experience that our people don't have."
He said the team from London would examine available evidence but not what he called baseless political accusations.
In October, Benazir Bhutto said her enemies in the government and intelligence agencies were plotting to kill her.
Asked if he had played any part in the assassination of his political opponent, the president said: "My family by any imagination is not a family which believes in killing people, assassinating, intriguing."
Ms Bhutto's Pakistan People's Party (PPP) has criticised the government investigation as woefully inadequate.
PPP Senator Babar Awan said the party would not back down in its demands for a UN investigation.
"Benazir Bhutto was martyred in a political conspiracy. The entire government is now involved in a save-the-culprit campaign," he said.