The inquiry into Princess Diana's death eight years ago was "far more complex than any of us thought", the man in charge of it has said.
Princess Diana and Dodi Al Fayed died on 31 August 1997
Lord Stevens, on GMTV's Sunday programme, also said it was "right" for Mohammed Al Fayed to raise issues about the deaths of his son and Diana.
He was asked by the Royal Coroner two years ago to investigate claims that the crash was not an accident.
A French investigation held driver Henri Paul responsible for the crash.
However, Harrod's owner Mr Al Fayed has consistently claimed that the car accident in a Paris tunnel was a deliberate plot to kill his son Dodi and the princess.
Lord Stevens, former Metropolitan Police commissioner, said: "It is right to say that some of the issues that have been raised by Mr Fayed have been right to be raised.
"We are pursuing those. It is a far more complex inquiry than any of us thought."
He said the investigation was being thorough as it looked "in minute detail" at the issues.
"We have only got a small squad doing this and a lot of the issues have been brought up by Mr Fayed and his investigations."
The car wreckage was being examined in the UK, and there had been co-operation with the French authorities, he said in the interview to be broadcast on Sunday.
"We have new witnesses, we are re-examining other witnesses, and, at the end of the day, I think what people want is a thorough investigation going where the evidence takes us, and some of the answers to some of the questions that have been raised in national newspapers and in other parts of the world.
"That is the job I have been asked to do and that is the job I'll do," Lord Stevens said.