The thieves left the note with the three paintings
Police say a disgruntled former member of staff could have been responsible for the theft of three valuable paintings from a Manchester gallery.
Detective Chief Inspector Peter Roberts said whoever carried out the theft had an intimate knowledge of Manchester's Whitworth Art Gallery.
Earlier, police displayed the handwritten note found with the the three works by Picasso, Gauguin and Van Gogh, which claimed the theft was to highlight poor security at the gallery.
The scrap of crumpled paper was found with the paintings which were rolled in a tube and left behind a public toilet yards from the gallery.
The note - a childish scrawl in black runny ink - states: "The intention was not to steal only to highlight the woeful security."
The method employed does suggest to me that they had an intimate knowledge of the gallery
Detective Chief Inspector Peter Roberts
The paintings - Van Gogh's The Fortification of Paris with Houses, Picasso's Poverty and Gauguin's Tahitian Landscape - went missing between Saturday night and Sunday morning.
Speaking at the gallery, Mr Roberts, challenged the thieves to get in touch.
He said: "If they see their actions as a noble cause, perhaps they are brave enough to come forward and give us their reasons."
He said it was a well-planned theft which required some inside knowledge of the Manchester University-owned art gallery in Oxford Road.
"The method employed does suggest to me that they had an intimate knowledge of the gallery and the security system employed at the time," he said.
Van Gogh painted The Fortification of Paris with Houses in 1878
The detective also said he believed the anonymous female who tipped off police in a telephone call is "closely linked, if not actually responsible" for the theft.
"We do not think it's a chance burglary," said Mr Roberts. "It's someone who carried out a lot of planning, who may know someone who works here, someone who used to work here or is an expert in security."
Mr Roberts said all past and current members of staff will be interviewed and hours of CCTV footage will be studied.
The museum has defended its security measures, which include CCTV cameras, alarms and rolling patrols.
The paintings were behind a toilet in Whitworth Park
But it did say that the theft had "highlighted a number of issues".
The three paintings went missing from the museum's Pilkington Room.
The Van Gogh suffered a small tear, but it is hoped that repairs will be completed within two weeks, and the paintings put back on display.