The man who launched Wednesday's attack at parliament was born in Britain and known to the intelligence services, Prime Minister Theresa May said on Thursday.
"What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism," she told lawmakers.
"He was a peripheral figure," she added. "The case is historic, he was not part of the current intelligence picture."
She said there had been no prior intelligence of his intent nor of the plot.