(CNN) Dawn Sturgess dabbed on what she believed to be perfume from a small bottle.

But the 44-year-old British woman was actually applying a Soviet-era nerve agent that had previously sickened a former Russian spy and his daughter, three sources briefed on the UK poisoning investigation told CNN on Thursday.

The bottle was the source of Sturgess' fatal dose, according to investigators.

Sturgess and her partner Charlie Rowley, 45, both residents of Amesbury, a town eight miles north of Salisbury, were hospitalized on June 30. Rowley remains in the hospital.

Charles Rowley and Dawn Sturgess are seen in images from their Facebook accounts.