Don, May may have said that.
As you say, she referred to potential radicalisees.

When it emerged that the suspects were known as would-be jihadists, it was asked why a closer watch was not put on them.

"Members will examine to what extent the alleged murderers were "on the radar" and whether there was any "culpability" on the part of the security service, the source said.

But they stressed: "There are thousands of people who are on the radar as would-be suspects. Only with a very small fraction of those are you able to provide that level of surveillance so that you know what each and every one of their movements is." "

http://www.independent.co.uk/news/uk/politics/did-mi5-blunder-over-woolwich-killers-we-will-never-know-8631565.html