There are currently no grounds to believe that failings by the security and intelligence agencies were to blame for the Westminster terror attack, the chair of the parliamentary committee which oversees their work has said.
The chairman of the Intelligence and Security Committee, former attorney general Dominic Grieve, also said it was wrong to think that arming all police officers on the parliamentary estate was a "panacaea" to preventing a repeat of the atrocity.
UK Prime Minister Theresa May's announcement that Khalid Masood was known to police and MI5 but regarded as a "peripheral" figure and was not under surveillance has led to questions over whether the agencies missed the danger he represented.
But Mr Grieve told BBC Radio 4's Today programme: "At present, there is nothing to suggest that there are any grounds on which his activities could have been stopped before he came onto the bridge in his car.
"Of course, I am sure that the security service and police will be looking very carefully at the history of this man, will look back at what they knew or what was brought to their attention and try to draw lessons from it.
"That is what happens after any such terrible incident of this type. My own committee, if necessary, is in a position to look at it as well.
"But I have to say that at the moment, I have heard nothing that suggests to me that it would have been possible to stop it."
Mr Grieve said that the agencies had been successful over the last 18 months in stopping around 12 attacks and plots.
"We have been extraordinarily fortunate," he said. "It has really been a miracle that it hasn't happened sooner."
Mr Grieve said he had "no doubt" that security at the Palace of Westminster would be reviewed following Wednesday's incident.
But he added: "It is worth bearing in mind that in fact security wasn't breached. There was a tragic loss of life but in trying to breach security, he was stopped."
In response to calls for all police at Westminster to be armed, Mr Grieve said: "The idea that arming all police will be a panacaea and a solution to this problem may well be mistaken.
"I am sure that all these things can and will be looked at again in the context of the Palace of Westminster. Everybody knows that the Palace of Westminster was a likely target, because it is iconic, because it stands for our democracy."
Mr Grieve said he was sure that there will be investigations into the possibility that Masood may have been radicalised in prison.
"There are plenty of examples of people being radicalised in prison, going in as a criminal and coming out with a veneer of ideology to justify their murderous actions and violence," he said.
"Clearly, the atmosphere within a prison and the extent to which prisoners can radicalise each other is something that needs to be looked at.
"Equally, allowing people to exchange views with each other is not something we can prevent within a prison environment and it wouldn't be right to do so."
The prison service has already taken steps to ensure that imams are available in jails to teach Muslim inmates the "orthodox" version of Islam, to counter the influence of radicals, he said.