All places open and digged

below is a paragraph from:
Jeremy Potter, Good King Richard?
© 1983, Jeremy Potter, used with permission
Chapter 13
The Princes: A Dolorous End
http://www.r3.org/bookcase/more/princes.html

begin excerpt
Yet with Henry, as with Richard, there is no real evidence and one
must suspect that if he had killed the princes himself he would
quickly have produced the corpses and an ingeniously appropriate
story implicating Richard. His frantic search of the Tower where he
had 'all places open and digged' may have been a deception, but his
conduct suggests that he never discovered what happend to the
princes.
end excerpt

does anyone know where potter found the quote:
'all places open and digged' ?

is it from a primary source document or from some latter secondary
source/chronicle?

thanks
roslyn