Pakistan's President Pervez Musharraf has said he is "not fully satisfied" with the investigation into the killing of opposition leader Benazir Bhutto.
But Mr Musharraf said he did not believe government or intelligence agencies had tried to "hide secrets" after Ms Bhutto's murder last Thursday.
UK Prime Minister Gordon Brown is sending a team of detectives from London to help establish what happened.
Ms Bhutto's party has called for a wider inquiry by the United Nations.
Addressing a news conference at the presidential palace, Mr Musharraf said that uncertainty remained over the exact cause of Ms Bhutto's death.
An initial government report said that she was killed when a bomb blast caused her head to hit the bullet-proof vehicle she was travelling in at the time.
"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."
For the first time since Ms Bhutto's killing, Mr Musharraf acknowledged that the crime scene had been hosed down, possibly destroying evidence.
"I am sure that they did not do it with an intention of hiding some secrets or that the intelligence agencies instructed them to hide secrets," Mr Musharraf told reporters.
"If you are meaning that it was by design to hide evidence, no, it was inefficiency."
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.
Men suspected of rioting make a court appearance in Karachi
Meanwhile, senior PPP leaders said police had been arresting party members across Ms Bhutto's home province of Sindh on rioting and arson charges.
More than 50 people died during violence in the wake of Ms Bhutto's death.
Elections due next week will now be held on 18 February because of, Mr Musharraf says, the damage done to polling stations and voter papers during the unrest.
Correspondents say friction with the government may increase during the run-up to the polls if the administration starts arresting PPP workers in connection with the riots.
The vote is seen as a crucial move towards democratic rule under President Musharraf, an important ally in the US-led "war on terror" who stood down as army chief in November.