(Reuters) - Dutch police arrested a 19-year-old Romanian woman on Monday in connection with the audacious theft last October of paintings including a Picasso and a Monet from an art gallery in Rotterdam, police said.

The woman was the girlfriend of one of two Romanian men arrested in Romania in January in connection with the heist, one of the most dramatic in years, in which seven paintings were stolen.

"Given the speed with which it was carried out it was clear it must have been well prepared," the police said in the statement in which they unveiled new details about the progress of their investigation, which has yet to locate the paintings.

Police were put on the trail by combing through security camera footage from the period leading up to the robbery, looking for visitors whose behavior suggested they might be making preparations for a robbery.

Security camera footage released at the time showed thieves entering through a back door and disappearing from the camera's field of view. Seconds later they reappeared carrying bulky objects and left the building by the same entrance.

Police singled out two men, Romanians aged 25 and 28, on the basis of their behavior and the frequency of their visits.

Police believe the unnamed woman, the girlfriend of the 28-year-old, was living in the flat where the canvases were stored until they had been removed from their frames and transported to Romania.

The other artworks stolen were by Henri Matisse, Lucian Freud and Meyer de Haan.

The investigation, which is being carried out in coordination with Romanian police, is ongoing.

(Reporting by Thomas Escritt; Editing by Jon Hemming)