Rubbish, it's a sash window which can be locked using string. He used the same window after the murders!
String ? You'd have to use something far less pliable in order to shift a sash window lock from the outside as besides the age of the locking system you also have the stiffness of new paint making the lock less efficient.
Sliding a penknife underneath from outside will open a sash window so I imagine it would be the same in securing it closed--------------but never string.